I hope I won't trigger an infinite loop by writing this. 
But yes, there is current software that would greatly benefit from it (media players, encoders, web browsing / flash, emulators, ...). They are current, and often used, at least when you use MorphOS as your main system. 
This doesn't mean the other points "against" Powermac G5 don't stand of course (consumption, noise, only a single core would be used on MorphOS and so on...).
I just strongly protest against this statement that no software would make use of it. 
Well, all of those apps/codecs are generally ports from linux and the like, so you could just use a cheap x86 box... wait... damnit! :roflmao:
Seriously though, these are the only apps that would probably benefit in that they aren't necessarily "fast enough" on a lower CPU. Of course, if one were to leverage GPU acceleration (even if only basic block decoding), the G4 is easily fast enough.
That makes the eMac perfect for MorphOS. Why would you wait for G5 support? Karlos is right. There isn't any current software that would greatly benefit from the extra processing power.
I know I made that point, but it's still cool to have the fastest kit you can run your OS on, even if most apps (save the x264 codecs) don't really benefit.