I think of it this way... If you are grouped with random people out of the DF then these people cannot read minds. Life becomes infinitely easier if when you just throw up a 1, 2, 3 kill order and at least mention to the other members that if they have CC then use it on the secondary mobs.

If you are on skype, vent, mumble, ts, or whatever then no marking will probably be as necessary. However as a tank myself I have all of my marks hot keyed and I can just throw them up while moving towards the mobs and have the entire pack marked and ready to go. Makes every single dungeon exceptionally easier. In addition to mapping out your plan of attack it also allows you to figure out if you have douche canoe dps and then you can let them tank after that point.