I have been waiting for someone to start this thread.
I find NASA'a official story on this to be hog wash.
The way I read their report, they are worried about a large piece of space junk in the vacinity of the hubble telescope. If there are problems with the first shuttle, the second will be launched to assist the first shuttle.
So I ask myself, Is the hubble telescope realy worth risking 2 shuttles and the crew aboard them?
Personaly I'm betting these shuttles have crucial millitary payloads that must be put into orbit before a certain date.