do they do anything besides look good?
Peavey Bandit 112
Boss DS-1 Distortion
Boss SD-1 Super Overdrive
Boss CE-5 Chorus Ensemble
Dunlop Crybaby Wah
Eastwood Ultra GP
beat up 1960's Gibson LGO acoustic
They give a little extra shielding from hum. I wouldn't advise removing them however. It's a little tricky, more trouble than its worth. Better off to get new uncovered pickups.
i heard they loosen up the bass a little and warm up your highs, nothing big and not vernoticable. but im not sure.
^A lot of players in the 70's removed the covers and started that myth. If you use an ocilliscope (sp?) you can see the difference but as you said its not very noticable.