I haven't tanked that long only around level 20, but it is a huge issue because DPS are squishy. By pulling a mob you are risking a few things.

1) Pulling a chained mob
2) The possibility of your dps creating enmity that outpaces Flash. (if you have a cross class ability like quelling strikes which lowers enmity it is not as bad). This usually happens when a dps keeps attacking like KHShadowrunner suggests.
3) And burdening the party by creating more work for both the tank and the healer.

At higher levels I don't see as much of an issue when you get high enough level additional skills like shield oath and sword oath can do wonders. I guess the main issue though is social convention, the tank wants to be the tank.