The reason its harmful is that it can cause tooth problems. It can cause hamsters to pull teeth loose, pull teeth out, or horribly pull a tooth out of place or twist it - and if the tooth gets twisted or out of place the ham will need its teeth clipping reguarly for the rest of its life.
Not to mention that all of those will be bloody and horrible and painful for the hamster
Might not happen, but its a risk I'd never want to take if avoidable - I know all of ours would hate having their teeth clipped and I don't want my hams bleeding badly because of something I should have stopped/prevented.
Oh and bar rub does happen and gets really bad if left. Our Scamp took some of her fur off and we were lucky - I've heard of hams who've taken their skin off with bar rub too. Do you really want to risk your ham bleeding and sore and painful because you didn't think its bar chewing was a problem?