But if this is purely a hobby machine, and you just like puttering around in CS2 and it doesn't really matter that the machine might fall apart at any moment and it doesn't actually matter how fast it goes, and so on... then what does it matter what you spend on it, if it makes you happy?

What motivates the purchase of something people use for hobbies or for nostalgia purposes tends not to necessarily make sense from a purely economical and numbers standpoint. I put a few hundred bucks into an Apple IIgs setup, but it's not because I expect the IIgs to be "the best" at anything I do, or even the most fun computer I have to use. 

If you view PowerPC Macs that way, then I don't think there's anything unreasonable in doing that.