RMS' belief system is that everything should be free software, and the GPL attempts to create (and maintain) a commons of such software.

The ultimate goal is not, however, accomplishable by any one license. And so the GPL is designed to enable this commons to be created and extended towards the point where his goal can be achieved.

If one agrees with RMS, then the GPL is usually the right license to use. (Sometimes he will advocate a BSD style license.) If you disagree, well the GPL is sometimes appropriate and sometimes not.

Cheers,
Ben