Tanks should only be tanking strong/elite mobs. DPS should be nuking down weak mobs.
Not sure about all tank classes but as Sith Assassin I have 3 AOE attacks (Discharge, Wither, cleave) I can open with plus 2 or 3 ranged attacks just to get initial hits on everything, no to mention taunt. At no point should anything be left alone to attack the healer.
Also there is no block/parry penalty for turning your back on mobs in this game so there's no problem running about a bit as tank to hit mobs.
I use Guard on melee dps usually