If there was a compelling collection of open source games I would certainly play them but that has never really been the case.
There are tangible advantages in having open source tools and components such as compilers,kernels and desktop environments even without every program on the system being open source.
Personally I do not derive pleasure from just playing games, I need to be able to study their source code and modify them. So I see no advantages in having proprietary games on my system. I hope that other GNU/Linux users acknowledge this as well. The one true value of the system is that it is GPL-ed.
If you look at places like /r/linux on reddit the Linux fanboys there are practically wetting themselves over steam.
I agree that more open source games would be a good thing, but I don't see it as necessarily more important than having access to the unedited recordings of a piece of music.