I disagree. It certainly does happen, and there is plenty of usable open source out there, and lots of patches from users that increased usability. The problem is that it's hard to find people with taste. But this is true no matter what development model is used.

Toss in a lower barrier to entry with open source, and you end up with junk at the bottom. I suspect that if you took the top N proprietary and open source apps, you wouldn't find a usability difference. The average would only get dragged down if you include the half-finished releases of open source stuff that would never have made it to market in a proprietary environment.