GtkRadiant is open source. There must be a lot of feedback and input from level designers on what features it should have. Radiant has a large base of professional users so we are listening to them very carefully when working on some new stuff. We didn't say it was pretty.
Community projects are a great way to catch a glimpse of the gaming industry.
Eventually you get sucked in and do that for a living. But at the very beginning you just do it because it's fun. I hear concerns that growing professionalism in mod teams might kill the fun of it, but I don't really believe it. When the game is in production they don't have much time to think about all the stuff the community will try to do with it. Then they get some time to think about it.