Well as I do polishing I feel I should put in my two pennies, but it's pretty much all been said.
If something is cosmetically past it then it is like giving it a new lease of life. Seems wrong to do it to something thats prestine and personally I wouldn't (although I would to a brand new piece).
I think bikenut2010 summed it up quite well when comparing it to classic cars. Personally whether it be a hotrod or a restoration I admire the amount of work and aesthetic beauty of both.
Anyway I'm just going over whats already been said.
Couple of my examples:
On my wifes Raleigh Twenty Special I did not mirror polish but gave it a good going over with the buffer removing defects and tidying it up:
http://www.retrobike.co.uk/forum/viewto ... l&start=15
The C-Record Crank on the Omega (still awaiting completition!) is mirror polished and looks fantastic in my opinion, the bike is very bling and it would be wrong any other way. It also has 11 speed Athena rings fitted as they are very nicely finished (although not engraved, just laser etched which will eventually wear off) Obviously the crank was in quite a sorry state prior to being polished:
http://www.retrobike.co.uk/forum/viewto ... a&start=15