The crux of the matter is thus:

Everyone should feel important in a party.

A tank who's only role is to just sit there and get slapped does not feel important.

They need to be constantly doing something that is worthwhile. There needs to be something they can work towards, that they can use skill to leverage and improve on.

Being slapped is not that thing. Especially with how encounters are designed in this game.

Thus, it comes down to damage. Damage is always useful, damage scales infinitely. Damage can be leveraged and the difference from a beginner Tank who's sat in Defensive Stance and spamming Enmity combos and the veteran Tank who's in DPS Stance and using DPS combos and rolling CD's where necessary is very noticeable.

Personally, I'm not too sold on Tanks being purely about personal damage output though... I've posited that it could be interesting to utilize a FFXIII style Stagger mechanic for tanks, where they work towards creating party DPS increases instead of focusing entirely on personal damage (It would still play a factor, but wouldn't be the only thing)