Hmmm, i thought i would easily find Richard Stallman's rant about the topic but it seems to have been toned-down somewhat. The Free Software Foundation do recommend using a slightly different version and a wikipedia article points out that CC-by-SA is almost identical
Just to contradict that a little chat in LWN about the issue
Apparently the GPL does mention the word "software" but only a couple of times. So it only needs a slight modification. Even a slight modification makes it a non-GPL though. Personally i think the best reason for not using the GPL is to keep all the licenses for all the official documentation the same so as to avoid confusion.
I think people want to get on with work rather than get bogged-down in chatting about legal issues which most of us have no idea about anyway. An ill-informed consensus decision can still be 'wrong' and land us in hot water.