To be honest... I really don't know how important fans really are. As a general rule it is better to keep electronics as cool as possible. Good convection cooling is probably sufficient (vent at the bottom and one at the top, hot air rises and draws in fresh cool air)... the fan is just one more gadget to put in the cabinet.
Everybody has thier opinion on this issue I'm sure... me, I just like plugs wires lights switches and stuff... and what the heck.. the fan surely is not hurting anything.