1. I wouldn't like this: having one type of gear for multiple classes helps a lot with gearing up

2. It is not only the tanks that share gear: Healers and dd-casters share their gear as well (as you are a Blackmage as your main(at least that is what your forum Profil says) you should be Aware of that)

the only bad thing about them sharing gear is that both classes need to have the same glamour, if that would be solved you could have your different looking tanks and People could glamour calls specific gear (like relict gear) without Messing up their look when changing to the other class.