Elsewhere, years ago, I said that because of the license I used, I may someday find that I have to pay a license fee to someone just to get a new variant of my own code. As it happens, that is starting to be the case. There is at least one company out there that attempted to keep a code base that contained my code proprietary. The only reason they had to release my code is that it was part of a code base that had been GPL'ed.
The GPL makes it so that you can't add your proprietary code and then restrict access to the new working code. New code added to a GPL codebase is also free. Other so-called 'open source' licenses allow you to poison a branch of the tree by embedding proprietary code. After that, it is no longer free software.
Years ago, I said that 'Open Source' was just the precursor to an end-run around the GPL. Now that the OSI had approved a variant of BadgeWare, that end-run is now 'in play'.
The GPL is well thought out to accomplish a single purpose -- protect the code of the world so that it can't be hijacked and imprisoned. There may be other licenses that do the same thing, but who are you going to trust? Why re-invent the wheel. I trusted rms to keep the GPL on track, and he did not disappoint me. I did not and do not trust the purveyors of 'Open Source'. Since I expressed my original reservations, the 'Open Source' label has deteriorated as I expected it would. Meantime, the GPL has gotten better.
Whenever I am able, I use GPL'ed code myself. The GPL is the only way to ensure that software remains as free as the day it was born.
AFAIK, Stallman is still a bit softer than I am, but I basically say that if it is not GPL'ed or truly FreeSoftware, it is a potentially poison branch.