My view - I am 100% happy with GPL for my MFD addons. I have had the privilege to resuscitate old semi-dead addons and breathe new life into them (e.g. Glideslope). I have taken much newer code (e.g. BaseSync) and added new features to it. I have created my own original work (e.g. RV Orientation). And I have partnered with cool guys to bring new ideas to life (e.g. Enjo with lots of things, but especially MMExt, and Keiththg on Lagrange).
Why have I spent thousands of hours of development time for free on these works? Fundamentally because I enjoy the intellectual challenge, and I like the sense of community in giving back something. Ultimately, its a sign of respect for Martins and the tens of thousands of hours he put into creating this fascinating simulation.
As for GPL - I cannot get excited about GPL, MIT, or other licenses. I only ask 2 things: (1) If somebody makes a derivative work from my source, then they keep the same freedoms for the next guys as I give to them, and (2) please don't expect or demand any support, warranties, or feature-enhancements (although I may at my discretion do them, or you may do them yourself if you prefer, or we could work together on them).
If anyone suggests that GPL is invalid, and has a bona fide case to complain, then we as a whole group can figure out what to do about it (e.g. make an Orbiter Public License!). But until then ... use GPL, release your source, and be good contributors. Recipe for life, eh?