I think from the designers I know and from my distant former life as a designer before I ended up as a CS guy, it only really comes down to one thing:
What's in it for a designer?
* It's not scratching an itch that they have.
* It's not great promotion (design has worked on a portfolio system for generations, so they have them already - showing someone what you've built is new and revolutionary for development in relative terms, but for design that's just how it is).
* It's probably not much fun (it's a skill to work with designers and to manage a project such that it comes out well - one that most OS projects do not possess if I had to guess).
* It's probably quite thankless (by no means a universal attitude, but plenty of developers think of design as an add-on, or a nice to have - not much of a situation to be in with your work).
So why would designers volunteer for this when they could do work for themselves and their portfolio that they may actually enjoy? How many projects will provide enough exposure to make this a good use of time?
I do design for open source projects, I've started helping out on the Gnome Project doing interface design.
• It's great for scratching an itch of 'I want to help other people'.
• It can be good promotion. The same way a good github account can help developers, stating and showing how you have participated on a open source project that is real is good for showing you can pull through on projects and deliver.
• It can be great fun to see something you designed being implemented and working. Even more fun if people praise you for your great solution to a problem. The thing with doing FOSS as a designer, you have to take alot of control - and push for developers to code your design. It's not just delivering a PSD, you have to work with the developers.
• If you solve a real problem, people will thank you.
Everything in your post is the absolute best case.
It can easily go the other way and your contributions can be ignored and never used, you will be reacted to with hostility by developers who think that design is an "extra" feature, and that even if you do succeed and something you do does hit a release, the overwhelming majority (nearly all) of people who use the software won't know you exist.
There are two sides to open-source development (at least, when it's not with people you know personally IRL) and one is, sad to say, generally not very pretty. Institutional hostility toward non-programmers still lives in a lot of open-source projects--usually the ones who need them the most!
(EDIT: Mind you, this isn't a reason to not at least try to contribute if you feel it's important to you, but the results are often not as rosy as your post suggests.)
You are aware absolutely everything you wrote is the same for developers too, even the programmer part;I've seen hostility towards people offering patches who are more users than programmers.
That's contributing to other people's projects period.
OSS is about scratching an itch, releasing code your business does but does not need to protect, or about self-gratification. While it can be a showpiece, you never get much in the way of fame out of it, no matter what you do.
Depends of if the designer has the same itch as the developer.
The software (at least in my perspective as both) is completely different and so the two problem spaces are rarely going to collide in exactly the right way.
What's in it for a designer?
* It's not scratching an itch that they have.
* It's not great promotion (design has worked on a portfolio system for generations, so they have them already - showing someone what you've built is new and revolutionary for development in relative terms, but for design that's just how it is).
* It's probably not much fun (it's a skill to work with designers and to manage a project such that it comes out well - one that most OS projects do not possess if I had to guess).
* It's probably quite thankless (by no means a universal attitude, but plenty of developers think of design as an add-on, or a nice to have - not much of a situation to be in with your work).
So why would designers volunteer for this when they could do work for themselves and their portfolio that they may actually enjoy? How many projects will provide enough exposure to make this a good use of time?