Another point: Sometimes you are not trying to damage someone when you hit.
An assassin just wants a chance at the death attack.
Some fighters are trying for a disarm.
And many spell ray attacks are "save or it sucks to be you".
There are magic items (that shield of devouring, whatever it is, that just kills an opponent it hits) that have effects dependant on hitting, but do not do damage as such, and thus do not benefit from damage as such.
All of these would be tactics that benefit from +1 TH, but not from +2 DA.