Marking a kill order is never a good idea.
Marking singularly high priority targets, such as bees, transformers, and summoners is a good thing because the marks are mechanically important.
Marking mechanically unimportant targets because you're not AoEing liberally trivializes the mark system in later encounters that actually have mechanically important marks, and results in the current group dying more slowly because DoT and AoE damage isn't being maximized. DDs that are not AoEing should be DoTing everything and then switching to the target that has the most health, as health bars fluctuate.
If the tank is geared and the tank is AoEing enough, there is nothing the DDs can do to pull things off him. DDs should take advantage of this and DoT all the things and target the highest health mobs.