Interesting note...
My wife read the post from Sass about isolation. She completely agreed that homeschooling parents often ensure there kids miss out on formative experiences in the public schools. She even gave a few examples:
Bullying
Gangs
Gunfire
Drugs
Contrary values
Formative experiences don't have to be positive ones. I do think you're exaggerating the gangs and the guns and (at least to some extent) the drugs, I don't believe most American schools are really like that, but this is beside the point. Out there in the world all of these things exist and you can only shelter your children from them for so long. In school you're forced to interact with a much wider range of people, many of whom are not very nice, and it teaches you a different range of social skills that will most likely come in handy in later life.
I'm actually not so sure that the educational attainment argument is the most important thing. I consider myself pretty well educated, and I went to a pretty good school, but in truth I probably learned almost everything I know just by reading a lot. I'm quite sure your wife and yourself can teach your kids to the standard needed to do well in their exams, and if you encourage them to be inquisitive and to read widely then they'll do better than a lot of their peers who are in the school system. I still think they're missing out on a lot though, and they'll become increasingly conscious of this as they get older.
Btw, I feel it's quite telling that you chose to cite 'contrary values' as one of the primary negatives. I rather confirms my suspicions about the main reason for homeschooling. George has a point of course that a small number of children really can't hack it in mainstream schooling and would benefit from being homeschooled, but I tend to doubt that many homeschoolers really fall into that category.