Case fans are good at moving warm air out(hopefully) of the case, but for certain computer parts, I'd assume they put a fan on it in the first place for very good reason =)
IE the cpu fan/heatsink. Granted there are probably better cooling methods then stock - but I can't think of a sane reason to keep on using the cpu if/when the cpu heat sink fan ever died.
Video cards may be a bit less fragile to heat then a CPU - but I personally wouldn't care to risk it - especially if a replacement fan can be easily attached in place of the old one?
Maintenance to prevent something from happening is probably a lot less costly and time consuming then maintenance after something bad has happened.. =/
-Jen