It's definitely easier for developers to contribute to open source projects, they can just dig into the code and easily contribute patches.
Designing for a project requires a lead to a greater extent to keep the design consistent, whereas programming is more defined and concrete work: "This is wrong, fix it." If the issue no longer occurs after the patch is applied, work is done. Design work is more abstract in that sense.
The designer's benefit would defiantly be in having the project in their portfolio. Additionally I don't think too many projects need an entire design team, for most projects one would be sufficient increasing the portfolio value you are talking about. Most people contribute to projects they use, adding functionality they would like to have themselves. I'm sure that is something that would motivate designers too.
I'm sorry but I strongly disagree (btw, I'm not the one that downvoted you; I don't have that option).
Why should I contribute to an open source project when I can do something like design a simple killer wordpress theme that may gain major adoption and use that as reference for my work than a single open source project catered to a single developers open source work that may or may not get used heavily. That or is only used in parts. There are tons of free designs out there, how often have you or anyone you know hiring designers reference how awesome or widely adopted their one off icon kit has been or something similar? Not happening.
Designers should contribute to projects they care about and use, adding the project to their portfolio should not be the sole reason to join the project. If you care only about portfolio, then designing a killer wordpress theme might be the way to go.
Designing for a project requires a lead to a greater extent to keep the design consistent, whereas programming is more defined and concrete work: "This is wrong, fix it." If the issue no longer occurs after the patch is applied, work is done. Design work is more abstract in that sense.
The designer's benefit would defiantly be in having the project in their portfolio. Additionally I don't think too many projects need an entire design team, for most projects one would be sufficient increasing the portfolio value you are talking about. Most people contribute to projects they use, adding functionality they would like to have themselves. I'm sure that is something that would motivate designers too.