I wish people would stop using fenders as they cut into my business...

Their chains and drive trains last longer and with a front fender a good headset will last forever as it keeps the tire from throwing water up the head tube and into the lower race.

With that being said... I run fenders on all my commuters.

From my experiences in the shop rehabbing old bikes I have found that fendered bikes tend to be in far better shape as those fenders have prevented water from infiltrating the headset and in the case of bikes with open seat posts, the bottom bracket.

It really reduces the amount of service a bike needs.

In the old days folks would put a cork in the head tube and seat post to prevent water from washing out the grease and damaging the bearings... I still do this on my bikes that do not have fenders and also do this for customers.