In Aurum Vale, indeed the reason might be to avoid luring too big packs.
But I saw this kind of behaviour in every dungeon, in every situation, with every mob. Even if there's a single enemy...

My POV is, the tank should run into mobs, and the melee dps should join him instead of the contrary.
During the one or two seconds the melee dps take to join the tank, the tank generates enough hate to lower the risk of a dps taking the hate and being targeted by a mob.

I realize there's an advantage, though : if the tank loses hate, it may be easier to be targeted again than if he had to run to the group...

But even like that, I find this a bit dangerous.
Besides, when I play a tank, I don't do that, and I'm always congratulated at the end, when I say I'm a newbie tank (what is true).