bushed turbos can be rebuilt without getting a new center cartridge. i rebuilt my turbo over a year ago, and it's still going strong.
some people also don't have the money to dump into a new turbo, so rebuilding is the cheapest, most effective way to keep the car on the road. At the time, I was unemployeed and needed my car functional so I could continue searching for a new job.
I bought my kit off of ebay. As long as it says it'll rebuild a garret T25/T28 turbo you should be fine. The T25 uses slightly stubbier bushings than the T28, but they have the same thrust washer, so most of the kits will include bushings for both kinds of turbos.
unfortunately, it looks like my best page i found on turbo rebuilds has gone to internet heaven, but these other links will help shine you in the right direction:
http://www.ecanfix.com/~mdhami....html
http://brickboard.com/FAQ/700-...g.htm