"You're using it wrong"
Richard Sapper, designer if the first ThinkPads, also designed an articulated desk lamp. Unfortunately it was found that some users might rest the head of the lamp against papers on tnekrcdesk, and the lamp being Halogen (no white LEDs in the 1980s of course) and hot there was a clear fire risk. So the manufacturers were forced to add a little stalk to the lamp head to prevent it touching things. Why do I mention this?
Well, the flat top of the G4 Cube was just too tempting a place for some people to place a paper document or cup of coffee. Users, hey? I guess the Trashcan Mac Pro's shape - also built around convection assisted cooling - discourages people placing folders on top of it, but might look like a nice place to keep ones coffee warm!
Steve Jobs had previously sold two other cube shaped computers, both designed by Esslinger (of Wega, Sony, and Frog Design), but they were aimed at brain surgeons (Pixar Image Computer) or whoever the hell the NeXT was aimed at. And all we got was Toy Story, the World Wide Web and iD Software's Doom.