Their manifesto starts by saying "The GPL is not a free license". This is because, in their opinion, "it grants different restrictions".
Let me state beforehand that I have nothing against non-copyleft free licenses. I even used them on a couple of mini projects of mine.
Now for the question: is the GPL a non-free license? In all my time as a Free Software enthusiast I asked this to myself often and at times I decided that no, the GPL was not a free license. Or at least that it was restrictive.
But I think the Occupy GPL website and the people sharing their view attack the problem from a different perspective than what the GPL actually stands for. Most of the times I read criticism towards the GPL (or copyleft licenses in general) it comes from the point of view of developers. That is, if I am a developer that wants to incorporate GPL code inside my proprietary software, I'm not free to do it.
Another quip from the Occupy GPL website is that GPL'ed code hinders potential collaborators. If a company wants to use GPL'ed code to develop their proprietary programs, they can't, so the original developer loses a potential contributor.
Both these counter-arguments stem from a premise: that someone would want to include GPL code inside proprietary software. That is exactly what the GPL was created to avoid. If proprietary developers find a GPL library useful for their work, they have two solutions:
- Make their own code free
- Not use the library
The mentality by which we should compromise our core tenet to please the proprietary world (and, thus, gain acceptance and collaboration) is what spun the Open Source movement. Sure, the free software community got a lot more contribution, but what actually happened is that proprietary companies took free sofware and used it to create walled gardens. The most infamous example has to be Apple. So yes, the free software community actually got more contributions, but do we live in a world that is more free because of this? I don't think so...
The point to remember is that Free Software is not meant to make other developers free to use the code. It's chiefly meant to help the users, who now have software that is guaranteed to be and stay free. The so-called permissive licenses are not meant for this goal.
In conclusion, if you think you may achieve more contributions and technical superiority by using a permissive license (which is debatable, see for example how the Linux kernel contributions and overall quality don't seem to be hindered by its use of the GPL), sure, absolutely go for it.
But if you care about long-term freedom in computing, I urge you to choose a copyleft license, such as the GPL, for your work.