NASA has a fully functional copy of Hubble "sitting around" at Goddard Space Flight Center as well. If something goes wrong in space, fabrication of replacement components and the training of the astronauts that will fix it does not occur in space. It is invaluable to have an exact duplicate on the ground for this reason.
Interestingly, the total 2010 US Space budget was $64.6B. The entire rest of the world combined spent only $22.5B. NASA's 2010 budget was $18.7B. Many programs that people think are NASA projects are actually defense projects. For example, the GPS system is not included in NASA's budget, it's spearheaded by the Air Force Space Command, and comes out of the Defense budget.
Chances are the main satellites that these are duplicates for have been decommissioned, so these are no longer needed. I would guess they are actually two distinct but similar designs, and not two copies of the same design. I would assume NASA already determined that the risk of these satellites failing and NASA being incapable of fixing them is outweighed by the desire to have higher powered telescopes in space.
My mother has worked in the thermal blanket lab at Goddard for years. Several years ago, she got one of the engineers working on the James Webb Space Telescope to take her and I on a tour of the clean room where they are fabricating one of the core components, the micro-shutter array. The micro-shutter array is an array of 65,536 shutters on an area about the size of a postage stamp. We got to go into the clean room and see the entire process. It is very similar to the process used to fabricate semiconductors, and I think they were operating at about the 60nm level. The idea of the micro-shutter array is that each shutter can be independently operated to shut out interfering light sources, so that the telescope can look much further back in space and time for deep fields. These should be spectacular. Instead of imaging the entire shutter area as the Hubble does, JWST will be able to close all but one micro-shutter which should allow very long exposure times, and the ability to see extremely distant objects. More on the array at http://www.jwst.nasa.gov/microshutters.html.
Also, the Hubble is huge. It is a cylinder with a diameter of perhaps 15ft and a height of roughly 40ft. Pictures really don't do it justice, I had no appreciation for the size until I saw it. I know my mother did some of the thermal blanket fabrication (think the tin-foil looking stuff on the outside of spacecraft) for Servicing Mission 4. Disclaimer: This is a cross-post of something I wrote at Hacker News earlier today.