Well, I never noticed my (or other) Warrior(s) showing "small numbers". At least for a tank

I honestly don't think its because one doesn't see the effects. The effects are obvious, people aren't dying, at least from the factors in your control. People don't like to tank because of the other members in their party and their attitude, the stress put on the tank since all eyes are on you (and the healer). I'm sure there are those that just want to deal damage, but I don't think showing how much damage mitigated is going to make someone jump up and start tanking, it would be nice for existing tanks though.

I honestly feel that "nothing" is really wrong with tanking, but with the players the tank needs to party with.