You can try and rationalize this to death, but at the end of the day it's an irrational decision. If you have a portable computer, it is irrational to buy an importable computer that is less powerful than the portable.
The 12" PowerBook may be an economical choice for someone who would travel with it more regularly than a MacBook Pro, but that's a different story.
To reiterate, I'm not attempting to be inflammatory. But I don't agree that it is objectively rational to buy an old and slow desktop computer when you have a newer and more powerful laptop. That is not to say that all decisions need to be objectively rational, or a critique. We're merely having a discussion about why people buy the computers they do.
i know the g4s are slow, but not to the point where their useless. i bought the imac g4 because i had always wanted one since they came out. i couldn't afford one back then. i just like having one because i always wanted one.
i collect sneakers too. all the jordans my mom wouldn't buy me when i was little, i have now. i've paid some ridiculous prices for some, but who cares? it's my money and i should be able to spend it on whatver i want. it doesn't have to be rational. it's my money. i like buying what i want. i live in an expensive studio too. i guess i should be looking for a place with bedrooms because a studio isn't good as house or an apt with bedrooms.
i use my imac g4 for web surfing and as a media server (streams to my ps3). sometimes i turn the screen to face my bed and i watch movies on it. it does those just as well as my mbp.
imo, i think the 12" pb is the ultimate portable laptop. this is my main computer. for what i use it for, it's great. my work got me a mb (to use with a lcd projector in my classroom), but i still prefer the pb. sadly, i cannot use my pb at work anymore. they don't want teachers using personal computers at work. i will be switching to the mb for work this fall. i also have a 20" imac g5 on my desk at work.
my mbp gets used for video rendering and heavy stuff like that.
i don't know what my point is. i just know, i like having my imac g4. it looks freaking cool. i'm typing this post on it.