I'd definitely agree with this if you want to buy a system to use Linux on - x86 and x64 are far better supported in general, even on Debian, and to my knowledge Yellow Dog has kind of abandoned PPC Macs as well. (Please let me know if I'm wrong, though!)
If you've already got the hardware, though, I honestly can't see why you wouldn't want to at least give it a try and see how it suits your needs. I've got my iBook dualbooting Leopard and Debian and both OSes run reasonably well.
Unfortunately, graphics acceleration under PPC Debian doesn't seem to be in the best state, so I've stuck to Leopard as my primary OS for the time being. I could see myself running Debian full-time if not for this, and TenFourFox definitely keeps things from feeling unusable on the internet, at least for the near future.
As for MorphOS... well, it certainly looks intriguing, and I'd like to try it, but I can't justify plopping down about what I paid, or in some cases several times what I paid for the machines themselves for an OS that I don't even know will suit my needs. (111EUR is about $150, last I checked, and I'm pretty sure people don't even value iBooks at that anymore! And there's no way I feel comfortable spending that much on an OS for a G5 I managed to grab for a dollar...) Not to mention, the licensing scheme makes me uncomfortable...