Well they will always do the right thing, and that is "kill the paper hero".
BUT tanky characters of course have their uses - let them be tanky just enough to survive, but pump up their attack damage as much as possible, use opportunist talent and be VERY aggresive, and I mean seriously charge them head on, if you manage to position your tank between 3 squishier enemies, they will take a lot of damage if they try to flee thx to opportunist - or they have to engage you, and that is the moment when you are good as tank.
If they still use abilites and such on your squishies, just focus their armor asap and knock them down. Also alternatively, your tank should probably have most of the support skills like fortify, magic shell and such to help your damaged teammates
