But it's a long damned game, and if you put anything important behind a dungeon completion it's going to be disenhertening - especially when you realize how many dungeons (and concequently runs) there are out there.
Depends on what you put there.
Personally, i find that dungeon completion is extremely satisfying, and a great 'sideways' direction to take the game if you find yourself bashing your head against the higher-level tasks like vicious dungeons and VGT. I completed the early dungeons 100% before tackling most of the harder content.
I think the fact that i am willing to get off the 'unlock treadmill' and play the game in a less-than efficient manner speaks volumes.
I would also say that many other players like this kind of play too, from the frequency of requests for more info on completion stats.
Putting something substantial, or at least permanently cosmetic to represent your achievement seems like a no-brainer way to get more out of the game. The last thing you want is players steamrolling all the way to VGT without playing with the vast number of combinations out there.