Profit is certainly not the only incentive that drives people. Saving time and effort by fixing problems (usability and otherwise) is also a great motivator.
In contrast, Closed Source products simply hire a professional interface designer if that's necessary to convince "no-users" to buy.
Apart from those, I would expect most projects would be happy to accept advice if the UID demonstrated competence.
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.