There's the example about putting every option in there as a user preference instead of making a decision or alienating a vocal user or developer. This even came up in the iPhone 4 reception UI "bar height" discussion: someone said to just make it a preference, and someone immediately said that's not the Apple way, which is true, but it's really not the way you design well in general.
For a designer to contribute to an open source project, there would have to be developers committed to implementing the work, to work on "polish" and "froofy things" instead of "real features" and other "important things." That's a hard nut to swallow, and what volunteer project owner will ask all their volunteer contributors to, please, stop working on your pet projects within this codebase and let's actually cut features and work on UI and usability and design?
Drupal is a great example. Acquia and/or the Drupal Association hired professional designers to redesign drupal.org two years ago but no-one in the community was interested in implementing the theme to completion or doing the work necessary to roll it out to the live sites. It might get done later this year, finally, as they've decided to actually hire people to do the work.
Drupal 7 is another example. Designers hired, but no takers on the complete implementation. Only portions of it made into Drupal 7.
I've personally dealt with this twice, trying to contribute new logos to open-source projects. One was using a trademarked character as their mascot (IP infringement), and they spent a couple of years discussing it before deciding to not do anything about it at all. The other spent only a year discussing it before it was decided upon, against the very vocal protests of a major contributor.
I would wager that as most of the developers of any given open source project are not representative of its end-users, the developers wouldn't even be involved in the design process in the first place. You can't design by committee, only take into account the needs of the stakeholders, and if you don't actually use the software, you might not be at that table. The designer would, essentially, only be generating work for them.
That said, I'm sure most junior designers would love to have "real clients" to work with, to get that vital experience, I'm just not sure many open source projects would actually be able to field them.
Designers on the other hand are known for their great designs that can't be reused. Their are styles/techniques, but rarely are they known by ego reference. Therefore, designers have a great ego interest in developing things designer praise - that's probably not esoteric open source projects.