Well, I havn't played my tank that high yet, but the Black Orc in particular provides a lot of buffs and debuffs that get applied to his defensive target when he does his damage abilites. Things like weapon skill increases, toughness increases and things of that nature. So as long as he sticks by the person that is getting hit, he can provide mitigation for some of the damage, making that taget easier to heal.
The Chosen, that I played briefly, had "curses" which are Point Blank AoE abilites that buff friendly units and debuff enemy units in the range. You can only have one up at a time, but when changed they have a residual effect for a few seconds, so it's easy to keep 2 up, and I've heard of people keeping 3 up.
The Dwarf tank has an Oath Friend, with which he shares damage and things. I have the least experience with this one though.
Basically, while thier damage may not be great, in PvP they will provide a lot of utility.
Edit: one more post and my post count will be 1337, lol.
Edited, Oct 1st 2008 2:51pm by Lathais
WoW - Smolderthorn
Barclande - 80 Undead Warlock
Lathais - 80 Orc Shaman