1. It exposes a programmer's code work for all to see. If it's bad, you get more than just critique. Others can point out what is wrong with the code but even better, they can correct the code or help you see what is wrong. If it's good or great, it makes for an amazing addition to any developers resume for hiring purposes.
The same can not be said of designers. Sure they can get feedback and people can often point out what's wrong, but these feedback are often more vague. Rarely do they get a walk through tutorial of how to do things better, actually having someone go in and show them how to fix their design flaw (something that happens a lot on the coding side). Pointing something out and having someone help you solve your design problem are not the same thing.
2. I don't know about others but to me, a designer benefits more from having their own portfolio rather than something that they may have contribute bits and pieces together for a project. As a designer, I don't see the value over my own portfolio which I can get enough critiques on without having to contribute to an open source project. As a developer, I see a need but think in terms of what designers benefit out of this. Open source designs are not the same as open source software in all cases. As an entrepreneur, I hire designers base on independent skill sets which is extremely hard to measure when people are co-designing small projects.
3. Just a comment but it would seem this is geared towards a developer who can't design and want open source designs but where is the benefit the designer is getting out of this. Surely there is a better argument. For the record, I'm not arguing that designers can't benefit at all. I just don't see it outweighing the benefits a developer would get in the same scenario.