Why a Hubble repair mission? Why not just launch a new one?

Today in space news: SPACE.com — NASA’s Mission to Service Hubble in 2008 Will Cost $900 Million

To me this begs the question: Why risk humans (and the fragile shuttle) to repair the ailing satellite, when for probably the same amount ($900 million) they could just launch one or more new Hubble-class satellites? The big thing about Hubble is that it is serviceable, but what if it wasn’t? Wouldn’t that make it cheaper?

It just seems to me that it would be smarter, faster, and cheaper to take that money and launch a couple of new replacements.

But this is NASA we’re talking about. Logic doesn’t seem to have a place in their planning when it comes to topics like the shuttle, the ISS, or the Hubble. *sigh*