I don't think staying at home with your kids is necessary, although I personally have trouble understanding people that make other choices :) However, if you are considering taking on the SAHP task/lifestyle, it is probably useful to know that it is a rather specialized sort of work environment.
First off, you have to be a person that gets enough fulfillment from looking alone at what you've done and being satisfied with it. If you need a lot of external validation, you'll be disappointed. Not only is the work undervalued by "society" (which abstraction turns out to be your family, acquaintances and friends), the days are so full of amazing moments that no one will ever know how smart you were, as there just isn't time to communicate the richness of a day. Notice that clever placement of a new toy near the front seat, so that it wasn't discussed during the car ride, and was then useful for distracting the toddler during the removal from the car? A totally smooth and on-time delivery of two kids for another event. The brief glow of pleasure from seeing the cleverness here is about all the positive feedback you get (aside of course from the smoothness and on-timeness). Also, it's the nature of the job that you really never finish anything, and there's often much unfinished work (piles of things sitting around, dishes that weren't able to be cleaned before dinner, dirty clothes, art projects that ended up with the walls being painted, broken artifacts that need mending). So someone evaluating you (that could be others or your own critical voices) would tend to miss the fun had, miss the conversations about how one can try even though they are afraid, and that that is what bravery is. They can even miss the fact of several hours being spent on cleaning because the abstract ideal of a clean house is not what springs to mind on looking around. So if you like results or like people noticing your results, it can be a long wait for either. You can only smile and move on to the next thing (or you know, grimace and try to see what happened and move on).
Secondly, a mother's work is never done. We've all heard this before, but consider this: you woke up with the kids in another lifetime; you've been tiring them out with nature and interesting stuff and cleaning up after ourselves and practicing writing by writing about the day; you've listened to insanely tedious squabbles about whose turn for what with this toy; you got a dinner that was eaten (lunch and breakfast while tiring don't even merit mention), criticized or not even noticed; you cleaned up while your help-meet "puts the kids to bed" (which may involve you stepping in at strategic times to ensure that teeth are brushed or other vital steps occur). Then as your partner mentions their terribly taxing day, the big deadline tomorrow, they drop off to sleep. Though you'd be interested in sleep, your knowledge of the effects of actions means you find yourself folding the latest load of laundry first. One bends into the task, knowing the long days are why the doubts as to whether you are making a difference with your life have gone, are why you sleep so soundly, having thoroughly used your allotment of energy for the day, and are why you just smile when you hear people talk about wishing they felt more fulfilled or connected to life.
Third, you are basically pooping on your paying career. As Anne Crittendon revealed, if your income is median, it's costing you a million dollars or so of lifetime earnings to pause for kids (if you were the "responsible" parent that missed work or left early when the kids had a pressing need, the loss of income for becoming totally at home is less, since you've been farting on your career all along). You don't even get Social Security credit for your labor, which is probably more intense and more effective than it's ever been before, albeit not as impressive to certain people.
Fourth, you don't get sick leave. So you have to have a constitution that allows the New England attitude of just keep putting one foot in front of the other. Because you will get sick with sick kids and things you can't miss. And you won't like it.
Fifth, you have to deal with a lot of poop - both actual and as a topic of conversation. My kids have this performance-art-esque dance "I'm pooping, I'm pooping, I'm pooping while I'm walking." (Quoted with permission of the author.) It can be very funny but you know after a day of leaked diapers and general disobedience, sometimes it's less funny. And really, is reviewing that "Daddy poops, Mommy poops, [sister] poops, [self] poops, [neighbor A] poops, [neighbor B] poops," etc., needed one more time? But as far as I can tell, it is needed one more time. Perhaps this talk means that potty training should take a turn for the more serious, perhaps it means time should be allocated in the schedule for changing a poopy diaper? These questions cannot be avoided. But they aren't glamorous.
Sixth, you do well if you are the sort of person that prefers to do stuff yourself. When I was a computer programmer, I was a low-level c programmer; I didn't want to use some poorly designed toolkit that would "make my life easier." I wanted to make my own perfectly suited toolkit from scratch. When I am responsible for a yard, I don't want some professionally manicured beautiful yard, I want to push my own heavy but low emissions reel mower and watch the clover fight the grass and the strawberries. With my children, I don't really have any reason to feel I'm doing a better job than the professionals would, but I am doing the work myself. I know what their childhood is like, and I'm passing on what I find is important or useful in living. And, I make the damn cakes from scratch.
Paradoxically, you also do well if you are the sort of person that immediately and graciously accepts help. All sorts of people will offer help, from clean shirts in the park when yours are all poopy or peachy, to watching kids so you can do various urgent tasks, to taking the kids out for ice cream while you bathe for the first time in days. There is always this temptation to turn down this help - not strictly needed, I want to save help for later, I've made by bed by leaving those peaches in the bag so long, let me lie in it; it's too late for a treat, we had treats today already, etc. You must be able to habitually resist that temptation, and accept the help without looking back.
Finally, the work can only really be enjoyed when you don't mind doing the unglamorous bits. In my professional life, I worked both in larger corporations where jobs are so specialized that I never even knew the people that touched the computers and also in small offices where I would do fancy programming, and as well would get out my screw driver and fix a loose cable. I always enjoyed knowing how the entire thing actually worked, from the most complex software process to the physical level of cables and electricity. With parenting at home, you are one moment attempting to explain death and why God doesn't really want us to worry about death so much as being kind to each other right now (in words that don't bore the question askers) and the next moment you are just wiping the butt cheeks. Even more grating to your ego will be when the whole family is going out together, and it hits you that your task to unobtrusively provide the "infrastructure for happiness." Your partner is regaling the kids with glamorous treats, while you silently hand over the wipes at the appropriate time. Your partner is expressively reading your toddler's favorite book, which you told her about and then went and got. You are at a restaurant to celebrate some job success, and you with no fuss bring out a little toy every 5 minutes or so to keep at bay the child-like behavior which our society frowns on. Again, you get to enjoy knowing that it could have been worse had you not been so cleverly prepared, as you flee from the restaurant, your partner's success not adequately celebrated, but celebrated as much as the universe allows today.
All this being said, I have never had more interesting work, nor work that so clearly gives me the chance to see my self and to master my self. Children grow up like their parents, and they mostly copy your values rather than do what you tell them, so the only way to parent is to be good, to be brave and kind and respectful and creative, even while you are tired and resentful and full of your own woe. Again and again, you have the chance, sitting there tired, to stand up and make reality better. It is leadership by example, and your kids will give you the chance to see how powerful that sort of leadership is.
