Yeah, tanks are extremely valuable in RvR, though a lot of the people who play them don't seem to use them right.... but that can probably be said for all the classes! Even besides the taunts, just the fact that you can physically block other players makes them really useful. I play a Shadow Warrior which is (mostly) ranged DPS which is really squishy so I totally appreciate having a tank in between me and the enemy. Another thing that's nice about WAR is that the classes are all balanced to work as a group... I'll nearly always lose a 1 on 1 with most other DPS classes, but with a properly working and balanced party, I can top the damage charts. Teamwork is fun!
Collision detection is so awesome it's not funny. As a warrior/priest, I can't soak up too much damage, but I can go and block for the ultra squishies.