I'm quoting these in reverse order because one kind of answers the other. Bouncing aggro around can be annoying for a healer, but it can also mean the difference between the tank going down, or a few entities being at 80% health simultaneously. Too many tanks think they can tank anything, they can't, so it's stupid to think that. Keep pulling and dropping mobs on the tank, eventually the healers won't be able to keep up. A dead tank when tanking multiple mobs will equal a dead group almost every time. Wasted mana or lowered DPS just means a little lost time, nowhere near as bad. A tank's first job may be to keep everyone else alive, but their second job is to keep themselves alive, or they will screw up the first job anyway. Know what you can handle, know what tricks you have and when they are running out. When that is happening, make adjustments, ask for help. People will think you are a crappy tank because the group keeps wiping, not because you couldn't tank 14 mobs at once.