He's right.
People need to tank and be a dd in order to see the 2 sides of the coin.
On low lv 15-20 dungeons , you can see that DD'S almost never wait for the tank to pull and flash, mobs and they always target different mobs,
I have been tanking and DD to lv30 dungeons, so i know how to play when i'm tanking and when i'm DD.
The problem is that many DD'S don't have a low lv tank to do some dungeons and know how stressful it can be on random DF.