In my fantasy world, that is how things would work. Bosses and trash mobs have very short memories. They're angry at whoever hit them last, and if they're getting hit from all sides, then they're angriest at whoever hit them the hardest.
It ought to be a regular occurence that a DPS with a good (or lucky) burst of damage wins the honor of being the primary target, even in the presence of good tanks. Because that leads to some interesting possibilities — or, well, interesting relative to what we have now:
- The DPS gets thwacked with a couple of auto attacks. Now, healers might have to spot heal someone who isn't the tank and who also hasn't made a mistake.
- The DPS gets nominated for a tank buster. Now, the tank needs to find a way to redirect or intercept the damage to themselves. Think taking a tether or PLD's Passage of Arms, but more creative.
What I'm trying to get at here is that there is an opportunity here to break up the tank-buster-and-raid-wide damage profile and to make tanking more than just, "lol, do (the right kind of) damage."