I don't understand why this "off tank" label is considered inferior. An off tank has equal if not sometimes more responsibility in other MMOs, the only difference between the two is tanking responsibility. There were times I would be told to off tank and time to be main, heck there were times that we ran content enough time that we used to switch between main and off, but never did I think I was inferior because I was an off tank. Some fights would require higher HP other times it requires dmg mitigation.

Some fights that I remember, fighting 3 bosses and having to keep them separated. Main tanks takes 2 and off takes 1, the key to not wipe is dependent on the off tank pulling 1 boss of the main after about 45 seconds and then having the main tank take it back in like 15-30 secs. At no point was one tank lesser than the other. Telling one tank to be main and off only meant who does what in the beginning.