- Better control over that damage dealt and its dynamics, likely with greater burst.
- The damage not coming at cost of potential mitigation.
- Auxiliary capabilities, such as mobility and offensive and/or defensive support. (Essentially, whatever the toolkit didn't spend on mitigation.)
I'm not saying tanks should have the same attack power and potencies, so to speak, as dps—merely that even if an OT were to deal similar damage over time, events could still bring out the need for a DPS through what parts of their toolkits aren't being spent on additional mitigation. Your tanks could be tanks in the mechanized, mobile, armored artilery sense of the word and the DPS "specialists" would still have use. This only increases as fights allow for greater player manipulation through their arsenals, and with Heavensward CC and such being about as bare-bones as possible that's pretty much the only direction of change possible.
Toolkits, not the trinity in and of itself, make things interesting. How much more damage are the damage-dealers doing because the tank is present, and why? In the tank's own toolkit, how does he maximize combined personal and (others') raid damage, both simultaneously and in trade?
Edit: to put it another way, unless the fight is specifically crafted to require a Tank's level of survivability (in a way that can't be countered through additional swaps, absorbs, or whatever else), DPS don't "need the Tank to be a damage sponge." Rather, the Tank being there facilitates DPS enough to be worth the trade of an extra DPS for that Tank. Now, depending on the shape of composition SE wants to push, the inclusion of a tank might almost always be a raid dps increase, regardless of their skill. Or, they might provide enough weight to the DPSs' auxiliary toolkits that there's actually some thought to that decision, and skill DPS can actually do viably well without a tank, say, on a typical dungeon or normal raid run. That's SE's balance to choose.
And with that, a sidenote: Would it really be so bad for something other than the 1/2/1 composition to be used once in a while, optimally?