Whaddya know, carsten, something we actually agree on!
I'm not a big advocate of homeschooling at all. I think it is only really successful if the child has lots of other opportunities to be around other kids, either in a church setting, ballet/karate/gymnastic, etc. setting, or through organized play groups with other homeschooled kids. It is likely, though, that a lot of the wealthier wizarding families used private tutors for their children, so the kids got a competent education, even if they weren't getting all the social development they'd need.
The thing that I find the most odd about the Weasleys is that Luna was apparently never a childhood playmate. Seeing as she was one of the only wizarding children in the area, and the same age and gender as the Weasleys' one and only girl, I would have imagined that Luna and Ginny would have been closer. Even if they weren't best friends, it seems like, at least in their early years, Luna and Ginny would have been playmates.
In addition to all you said, I think this trend towards homeschooling had another effect: it enabled families like the Malfoys to keep their son away from "riffraff", therefore creating just another snob. Malfoy, I would assume, knew Crabbe and Goyle before Hogwarts, so it's possible, since all their fathers were friends, they were playmates since they were small. But if the Malfoys either taught Draco themselves, or had a private tutor for him (which I think is more likely in their situation), they had control over who he was around all the time, and they were able to keep him away from "trash" such as muggles, mudbloods, or wizarding families like the Weasleys.