In 3.0, <lv58 Tanks were having hard time dealing with all the mobs and getting wrecked, which left barely time to go into Cleric Stance to put a DoT or two.
I started DPSing as Scholar when my DF Tank (JP) told me it was fine specifically to not heal until down to 50% HP or so.

Up til then I've been keeping tanks topped up >60% because I had random tanks who complained about healers being crappy when their HP comes anywhere near 50%.
Also EN players I know complaining about healers letting HP drop frequently.

So I feel that its more of a default attitude to not DPS to avoid criticism from touchy tanks about their HP.