Just no Bootcamp on G5's
That's not what I use my Macs for 99.9% of the time (hardly ever touch Boot Camp even on my MBP. I've got a real PC for gaming).
OTOH, my upgraded PowerMac G4 can boot into OS9 baby! Yeah!
Ok, that's not terribly useful to me either, but it's kind of cool to show some friends. I have older Office and Photoshop versions for OS9 and I can still print to my Brother dual-sided Laserjet from OS9 as well. Some older games will run in it as well, but for some reason my CPU upgrade mungles the sound most of the time in OS9 unless I re-install the boot header before I boot, but then it goes away on the next boot again so short of playing a game I don't have for Windows for that era, there's not much point anyway. Running an ancient version of iTunes is kind of fun, but most of my collection is MP4 not MP3 so I only have a couple hundred songs I could play anyway. I did like the Window border gadgets in OS9, though. They were kind of slick looking. But OSX functions miles better overall. My upgraded GPU doesn't have acceleration support in OS9 either, so the Rage 128 that came with the PowerMac was a lot faster in OS9 as well, but it does still boot and takes up almost no room even with a load of software so I keep it around.
In any case, the biggest limitation of some of these older machines is that Apple isn't supporting them in the OS anymore. HD video runs slightly below smooth here (not normally a big deal since I watch movies downstairs on my 93" projector based home theater not in my den, although I do have some pretty nice speakers connected to it in the form of Klipsch ProMedia 2.1, which I also use in my bedroom), but otherwise, this PowerMac can do most of the stuff my MBP can do (save Logic/FCP although older versions would work) and I have it on 24/7 powering my whole house audio/video system based off iTunes, Apple TV units and Airport Express audio only receivers. I have audio in 5 rooms and video in 3 and haven't used a CD or DVD other than to load them on the system in 3 years now. Until iTunes no longer supports Leopard/PPC or the system dies, I see no urgent need to replace it. It gets used 10x more than my MBP since I use it for most if not all my typical daily surfing, e-mail, etc. (since it's always on I just turn on the flatscreen monitor and I'm already there).