Pretty much. High tier tanks can probably get away with going an entire dungeon without entering tank stance with the way most DPS play in DF, but that flies out the window the moment you run into an equally high tier DPS. Especially if they're a Bard that lacks Diversion. ;_;
(I've run into a grand total of one tank who was able to keep aggro away from me without ever entering tank stance, and it was a Warrior, by far the easiest tank to pull that off as. And I still had to dump aggro frequently, though most of it also consisted of 'okay gotta kill everything before I rip aggro and this becomes a problem'.)