Well, Ive never owned a desktop but so I had to go with the 'only on a laptop' option. Even if I did have a desktop, I'd have to give applecare a good long look.
Didn't buy it for my G5. I did buy it for my PowerBook, and good thing I did because about 2 weeks after the standard 1 year expired, I had to get the inverter board replaced, so AppleCare paid for itself.