One reason is that corrosion, etc. can build up on both over time and and that actually puts more stress on the ignition coil and can even cause the alt. to work harder.
Even if they look clean it's still a good idea to replace them at some point if you know they have a lot of miles (more importantly, revs) on them.
It's similar to why you replace the plug wires if they're real old, even if they don't have cracks all over the insulation. There's a lot of "juice" flowing through that stuff.
My cap and rotor had burn/arching marks on them and lots of corrosion. Even after I cleaned up the corrosion I decided to get a new pair a while back and once they were on I noticed significantly less noise from my ailing, 1/2 dead alternator.
Last edited by A-series; 09-15-2007 at 02:13 AM.