I personally agree with the philosophy of the author: I think all software should be free, and in the perfect world, I would license everything I do under the GPL. However, not everyone in the open source world has this philosophy. A lot of people prefer more permissive licenses like Apache and MIT, and using the GPL excludes these people and projects from using the code - by its viral nature, any single usage of the GPL would force the entire project to be under the GPL as well. Cooperation is the greatest strength of the FOSS world, and so I'm willing to compromise a bit to make that possible. As such, I personally prefer the MPL - it is a weak copyleft license that enforces all modifications to the source code itself are released back (important for me), but unlike the GPL is not viral and will not spread outside its own source files to other projects (important for others). This way I can keep the project and all modifications to it free, but also allow it to be used by others who don't share that point of view.