I've not yet found a reason to replace the BB in any of the bikes I've done yet, and they've all been cup and cone. Every one has had nice smooth bearing surfaces, spindles have been in good shape, etc. I did by the Hozan lockring tool and my BB service became a joy. I remove everything but the fixed cup, remove all traces of old grease, grease 'em back up and install. I've yet to have one not be smooth as buttah. BB service is all about the tools to do it right and to set the compression on the cups properly. If I ever wear one of my own out I'd probably replace it with a Shimano sealed BB.

I have to imagine that cup and cone BBs will last a DANGED long time properly and regularly serviced. Now that I have this ultrasonic cleaner my bikes will all get torn down every winter (I can only have a maximum of 5 so we're not talking a huge stable)...including the BBs.