Anyone who overlooks a mesmer in the tombs when they don't already have one is an idiot. A mesmer can bring about a quick victory by disrupting enemy casters or simply sapping all their energy.
A good team ranks mesmer up there in the targetting. Heck, often times when I play my mesmer I get attacked before the monks. They know I can't really heal myself and if they spike me fast enough the monks can't save me.