The primary reason is that once the Stable version of Debian is set, so are the packages that come with it. Even though they get security fixes and bug patches, you'll be stuck with, let's say OpenOffice 2.0 when others are “enjoying” version 2.4.
Read more thoughts here
With two hard-drive bays and less worries about ACPI, the G4 is a great test bed for anything and everything that runs on a PowerPC chip, and that frees up the Gateway to be more of a working computer with a lot less dual- and triple booting.
While on that subject, I'm moving toward chainloading everything I can out of the main GRUB in the master boot record. That way I don't have to rely on an update to one distro in a dual- or triple-boot system to rewrite /boot/grub/menu.lst in every installation on the drive.