Powermac also can take much more RAM. It'll probably be more quiet...though I can't say this for sure, I haven't seen the iMac G5 at all. And by the way, the bus of the Powermac actually goes up to 1.25 GHz. So to update Mac Mogul's comparison:
iMac G5= Single processor, not capable of 2 ghz, and a bus speed of around 600mhz. 2 gig max of RAM. Not so expandable at all internally.
PowerMac G5= Dual Processors up to 2.5ghz with bus speeds up to 1.5ghz each. 8 gigs max of RAM. Expandable enough internally.
Basically, the only real reason at all is cost. And of course form factor.
I personally wouldn't get one myself...I'd actually choose a G4 Powerbook because if I wanted something for it's form, and was going to sacrifice expandability, I'd sacrifice the G5 processor for the laptop form factor.
Also, you can cut costs off of the G5 Powermac's final price tag by not getting a display from Apple. Sure, they look good with the G5, and they're actually really awesome displays, but unless you have the money to burn, I don't see any advantage to them over a conventional LCD that justifies it's price. We can do the cost justification for the Macs themselves almost entirely because of OS X, but not for a monitor.