I take them off all of my bikes, but put them on bikes I build for inexperienced people.
I went the 'whole hog' on this 1980's Claud Butler Elan I built for a novice rider, a young lady doctor who wanted a Classic 1980's English bike to commute four miles to the hospital in Hamburg, Germany.
As you can see, it's got a metal dork disc, chain ring protector, reflectors on the wheels and pedals (and by the time it was sent to her a bell, front and rear reflectors and mirror), CLB turkey levers with Emelle stem shifters for a 'heads up' riding position in traffic plus robust Shimano Tourney derailleur. Locked in the hospital bike rack for more than 12 hours a day it's likely to get knocked around a bit. The last thing she'd want after a long duty is to have the chain wrap up and have to push it home. As it is, she loves it! If I'd built it up for myself it would be very different.