Free speech: I can write a book, and charge money for it. My speech is copyrighted. I have freedom to speak, but I also have the freedom to charge people to use my words. GPL takes away that second option (somewhat - the analogy isn't perfect).
The GPL makes selling software difficult. Any software you create using GPL'd libraries, you have to distribute as GPL. I am making a commercial game, trying to start an independent business. Games are sold. If I use a GPL library in my game (say, for a tiny piece of URL resolving, or PNG file loading), my entire project's source code must be distributed for free. I can still charge for my game... but anyone else could compile my game and sell it as well (if they replace the art and change the game's name). That's not good, and makes GPL'd software worthless to me.
I get that modifications to the GPL'd code itself should be released, but I heavily disagree that my entire game project should be forced to use the GPL because 0.01% of it relies on a library that's covered by the GPL.
LGPL resolves this problem (if you dynamically link), so I'm heavily in favor of LGPL code.
I think GPL is fine for entire projects, I don't mind using a GPL'd art program, for example. But imagine if using that art program forced the art you created to become Creative Commons license (thankfully, they don't). And imagine that you are trying to make a living selling your art. It'd make that art program unusable to you. That's what GPL libraries do to your code.
GPL = Fine for entire projects, utterly horrible for libraries (unless you don't want your libraries used for commercial projects).
LGPL = Perfect for libraries, not as great for entire projects (depending on the desires of the project).
GPL is designed to further the open source movement, by virally infecting anything that uses GPL to make the using code also become GPL. It's too demanding.
LGPL is designed to force you to share changes to something someone else made (good), but allowing you to use that something to create something new, and still lets you retain control of the new thing you created.