Was this fan absolutely positively necessary for keeping my PC from exploding?
Did it explode yet?
If not, then probably not. ;-)
Seriously: it kind of depends on your PC -- what kind of CPU, video card, how many harddrives, whether or not there are any ventilation holes in the case, etc.
I have a 400MHz, 2GHz and 2800+ machine that all ran just fine at acceptable temperatures without any case fan... but I wouldn't want to try it in a system with a 3GHz Pentium4, killer video card and 4 Maxtor harddrives....