Firstly, open source does (or can easily) have one big usability advantage: installing things, assuming it's on a repository, is much easier. Or, at least, it's very close to there; I haven't asked a lay user how useful they find the "Add Application" menu in Gnome. But assuming it's not already done right, once it is, that's a huge advantage.

Secondly, so we've admitted that non-company-backed open source projects can have good code, making for a good backend. So the economic model does work. What we need now is for more non-developers (UI designers, artists, etc) to join this movement. I think that it's slowly happening. Tech savvy ones would be more likely to join, and as it becomes more and more inviting I think the less savvy will join as well. I can't think of any specific reason this economic model applies to developers more than others, so I think that over time, UI will become good for the same reason the code is good.