I think this is an entirely fair point to make. My British babysitter joined our local brass band and presto -- she had a whole load of local friends who shared her interest. And she was committed to making the whole integration thing work. That's the key.
I, for one, have no hobbies to speak of and if I didn't have the dubious benefit of a non-English speaking Kindergarten teacher breathing down my neck, I'm not sure I would have learned much German. If my kids went to international school, I'm sure I would have been a fully-commited member of Expatlandia and be done with Swiss society other than the most tangental and superficial relationships. But that's just me.
If you want to make the whole integration thing work, you will, regardless of the situation. You just have to be honest with yourself and figure out what you really want.
The thing about sending them to local school is you will have an instant bond with people you probably wouldn't associate with normally. Much like new mothers will happily talk about the colour and consistancy of baby poo, the same goes with fretting parents of school-age children. Swiss people bitch about the educational system just as much as foreigners. And over a couple of glasses of wine at parents' evening, the differences tend to melt away. But this can be achieved in other ways. Its just harder.