Freedom vs. Freedom
A little background
The GPL is considered to be the license that guarantees the freedom of your code for all time. If you use GPL'ed code in your program, you are legally bound to release the code to your program under the GPL. The idea is to protect Open Source code from people who would take it and build a commercial venture around it.
Then you have the other Open Source licenses like the BSD license, the Apache license, and the OpenLDAP public license. All these are also Open Source licenses but unlike the GPL, you are not legally bound to release your code under the same license. What they do say is give the authors credit, retain the copyright notices and include the license in their software distribution.
Ideology is important ... so is access
I firmly believe in the power of Open Source because this is the only thing that will prevent monopolies like Microsoft from coming up. Microsoft may steamroll a corporate entity, but how does it combat the diverse Open Source community?
Open Source is, for want of a better word, open. You see the code, the very core of the program. It is available for you to modify, extend and more importantly learn from. No one can deny what a fantastic opportunity for learning Open Source provides. Knowledge is power and Open Source hands you the power.
I don't think any of the Open Source licenses deprive you of the right to view and modify the source code. What then does the GPL offer in terms of access rights? I can contribute just as effectively to any of the BSD licensed projects and you will find a large part of the Open Source community working on such projects.
The moral high ground - "Free as in free speech, not, free beer"
This brings me to the moral high ground that GPL and GPL supporters stand on. The argument is that the GPL prevents un-ethical behavior by preventing an entity from grabbing the code and building a proprietary program out of it. So one sees the GPL as the guardian of the electronic equivalent of Free Speech.
The Everybuddy case
Let's look at the Everybuddy case. (URL) DSF Internet took a GPL'ed program, built a Windows port and started selling it to clients. Oh, and they conveniently forgot that they were legally bound to release the port under the GPL too. This was a GPL violation, which would have gone undetected, but didn't. DSF Internet was subsequently forced to release their code under the GPL.