The GPL is a big bait and switch for the following reasons:
1.) The copyright holder can change the license at any point to a proprietary one if he or she chooses while someone who uses the GPL code (non copyright holder and average user) is bound to the GPL.
2.) The copyright holder forces contributors to use GPL compatible licenses, but can take your contributed code (after it has been converted to the GPL if you use MIT for example), and change it to proprietary if necessary. Recent example is the Mir project that is GPL, but has intentions to make the code proprietary if a phone carrier requires it.
3.) The copyright holder who wants to make money from offering GPL code generally comes with a paid support contract tied to it since paying for the actual code is not required with the full source code available.
4.) The copyright holder can also make money by dual licensing between the GPL and a paid for commercial license to take away the copyleft. In the case of the Affero GPL licensed MongoDB, they lure you in with the free code but if you want to make custom modifications to the code for your company, you are forced to release the code since the AGPL requires code for it run as a service.
Of course 10gen offers you a way out with a paid commercial license so you can have actual freedom to modify the software and at the same time they can make money and keep their company afloat. http://blog.mongodb.org/post/103832439/the-agpl
There are trade-offs and benefits from using a free software license, but do your research and be 100% sure in which one you want to use if you are looking to develop under one. Many companies have opted for the non-copyleft permissive licenses because they may choose to integrate it with proprietary software or just don't care what people do with it as long as they use it.
I've said this before in my endorsement of the Mozilla Public License 2.0 in that I see it as a great bridge between the GPL and permissive licenses. I feel that free software projects can gain traction with this license in preserving the copyright and not scaring away commercial entities by forcing the non-copyleft code to be copyleft like the AGPL and GPL.
With the MPL 2.0 being a file level copyleft, why do I care if it is used with or linked to proprietary software as long as my MPL 2.0 licensed file is kept pure? If I used Apache 2.0 or the MIT license, they could still link with proprietary code but at the same time have the option to make my work proprietary without my consent. That is not an ideal situation for me as a developer and I feel more should give the MPL 2.0 a chance before licensing under a pure permissive license.
