You can't cover every situation with a sweeping opinion that marks are always unnecessary.

Sometimes, other players simply don't know the dungeon as well as you do. Another potential issue is a well-geared over-zealous DPS throwing himself at mob#2 as soon as you pull a pack and start on mob#1 - before you've even had a chance to rotate or flash etc. No matter how good you are, that's always a possibility.

At the end of the day, it takes less than second to mark (you can even link it to SL with a not-so-complex macro). As tanks, we naturally end up leading the group so if marking up clears confusion for 1 individual player 20% of the time, then you've made a difference. You've mitigated the mess & confusion, saved yourself having to drag a pack into the casters to pick a mob up, saved the healer having to heal said DPS - you get the picture. It's the easiest thing ever, why not?