Increasing the party DPS by 200 is huge, especially when you're doing it with a non-DPS job. It makes a big difference. And I don't think 200 DPS is 10% of your damage. Now imagine if the two tanks in an 8-man party are doing that, and add that the healers might also do it. That's huge and it helps a lot meeting hard DPS checks. Even with the best DPS players in the world.