Why wouldn't someone want it? It's still a perfectly good computer. I wouldn't pay very much money for one, but if given one for free, I would happily use it. (It could be easily upgraded to be better than my current desktop, a 1.25 GHz eMac.)
My main computer is a 1 GHz G4 and I do everything with it. I run Tiger, iLife, internet, music, burn CDs and DVDs. The only thing I really can't do that I'd like is to watch high definition movie trailers.
When I went from an 867MHz G4 (Powerbook) to a 1.5GHz G4 (also a Powerbook), the difference was fairly subtle for most types of use. Even going from my G4 to the current Mac Pro, basic things aren't shockingly different. IMO a G4 Powermac, with its expandability, is still a fine desktop. As a matter of fact, if I didn't need to run Windows apps for work, I may not have bothered to make the Intel Mac transition at all. That said, I do need to use Windows for work, have bought two Intel Macs to date, and am looking at a third (iMac for my dad).