I'm still working on tanking and I am getting really close to level 50 on my first class. In my opinion marking is something that can be done at lower level, but shouldn't be needed in most cases. I've been doing Tam Tara on Low Level Roulette and had groups that insisted that I needed to mark for them so they could understand what to attack first. In an attempt to help smooth the run out I started marking ever fight. Nevertheless they STILL didn't attack in the right order. Though I go above and beyond with that by reminding them to attack the mobs in the priority order, a kindness which is sadly overlooked on many occasions. Sometimes after all that they go out of their way to make sure to attack in the reverse order I marked. So I think the problem isn't tanks not wanting to mark it is that there are tanks out there who refuse to mark because they know it might not change anything. Also before I get a rap as a bad tank some of them had classes at level 50 and should know better.