Well, I had to go over to the homeschooling board to check out the conversation over there that led you to posting this, Dar, but now that I've done that, I can answer

: :LOL
I checked everything except for the one about everyone else sends their kid to school... But I have 2 kids (11 yo foster son, and 20 mos old ds) and different reasons apply to each of them.
For dfs:
I go nuts trying to explain things to him. He has some bizzare learning disabilities going on, which, when combined with his attachment issues (which are more present with me, who he has to learn to accept and love as mom, than with a teacher who is just a teacher), make for very unpleasant times for both of us. He freezes, I get frustrated, he refuses to ask questions, answer questions, ask for help, accept help, etc. etc. Its ugly.
He and I do do better when we're not together all the time. Again, this is due to his hx and behavioral stuff. I get bitchy with him if I'm around him 1:1 all the time. He also is used to and thrives in very active environments. Home with mom and a toddler doesn't cut it.
he does need the social group practice, having lived in a residental home with other kids w/ severe behavior disorders. He really needed some full immersion with typical kids, in order to adjust his reality of how kids behave and how adults respond.
(I'm trying to remember the poll options here, so I can explain for each of them, but I'm failing..)
Well, for ds, who at this point we plan to send to school (although, I do think it is foolish for anyone, myself incl. of course, to "know" what type of education will be right for their child when said child is still as young as mine...) mainly b/c we do feel strongly about supporting the public school by sending our children their, and then being active involved parents.
I do have one friend who has been volunteering at the public school w/ her son since he was 3 ish. Her dh is a public school principal in another town. They have chosen to send their ds to waldorf for K, but she still will devote lots of time to the public school. I really admire that. Otherwise, I see that in my rural community, our school is really suffering b/c of parents choosing private schools (driving their kids a min. of 1 hr round trip, some up to 2-3 hrs each day -- why not spend that time at the public school, improving things for all kids in our community??
Also, just a side note -- in the thread on the homeschooling board, someone made the comment about handing their kid over to a complete stranger for K. Well, in my case, and in my ideal world, where you are *involved* with the school, its not a complete stranger. One of the K teachers is a friend of mine, and the other I know well enough from time spent at the school to feel very comfortable with.
Oh, and the other reason I send dfs to school is that foster kids have to go to school. I wouldn't be allowed to hs him even if I wanted to.
Okay. Sorry for the mini rant. I really don't have anything against hs'ers or unsch'ers. Like I said, it may turn out to be what is right for my ds, and then I'll have a lot of crow to eat, cause I spew this rant on a regular basis.

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edited to fix a spelling mistake that I now can't find! somewhere there is a their that should be there (or maybe it was they're, now I can't remember, but really, I do know the difference!!)