They are a total pain in the a$$ IMO. I have them for my '74 P and absolutely hate them! The ONLY reason to use the bridge cover (there is never a reason to use the PU cover IMO) is if the rubber mute is required to deaden the sound IMO. Beyond that, people must use them 'cuz the look cool!
Generally they're only cosmetic on newer basses, though I like using mine as a guide for my hand whenever I slap.
They are purely cosmetic these days, but they do look bitchin! If you have a P or J, seriously consider getting these. Also, you can use shielding tape on the insides of the covers to further reduce hum.
they seem to contrict the magnetic field. I recall my bass sounding different with them on. not like a 'forget about getting new pickups' change, but definitely not the same open tone. I loved it on my neck pickup, hated it on the bridge.
i have the bridge cover on my P and love the look of it, dont use the pickup cover as it gets in the way, i play right over the pickup. i love the look of both covers on my J, but are more in the way than the P. i took off the J neck pickup cover, but it looks kinda goofy with just the bridge ashtray on it. i wish it didnt!