My personal systems run Win95 and Win98. The memory gets more corrupt with each program close. I open and close many programs when I use the computer. Thus if I don't shut down everyday, it will crash on its own. The longest I ran was about 2.5 days. Sure I could run RC5, Seti@home, a server, or something else that runs constantly - then I wouldn't close the programs and wouldn't get the corrupted Windows. However, those certainly are not good tests of a computer's stability. Plus the cost of electricty outweighs the potential benefits to me.
At work, I leave them on for weeks or months at a time. That is how long it takes to get some of my problems solved. I run Win2k to avoid the Win9x issues.