I don't, whenever possible. I certainly never use GPLed code that I know I'll need to modify later (for example, I probably won't ever modify the MongoDB source code).

It's also the reason I stopped hacking on Diaspora after the initial excitement wore off and I actually read the licensing terms. Well, that and their ill-advised copyright aggregation policy, which was apparently set up to allow for future dual-licensing (which I thought was pretty hypocritical).