I disagree. The main tank is a constant source of outgoing damage with little to no opportunity cost; it's a reliable source of damage to optimize. DPS and the off tank take priority, but the main tank is just behind them. Besides, even if it weren't an effective option, it's still an option.
First of all, the only "mitigation" you can stack is Parry, and that's not reliable. VIT can provide enough buffer HP between incoming damage to mitigate the opportunity cost of switching to Cleric Stance, but you need considerable amounts of HP above the minimum HP for that (I would say at least 25% above). But at that point, just how much STR are you sacrificing?Meanwhile, a main tank who's stacked on mitigation can spend enough time not being healed that the healers can sling a few damaging spells of their own out.
I don't understand why some people continually insist that it's not the tank's job to deal damage. If that were the case, then it wouldn't deal damage. Instead, damage occurs naturally while tanking (as opposed to healing), and in considerable amounts. The main tank will generally deal 40-60% as much damage as an equally geared and skilled DPS; that's not negligible.The job of the tank is to keep the attention of foes and, more importantly, not die. If the tank dies, so does the party. Doing as much damage as possible is not in that job description, and attempting to do so only makes your actual job harder unless you appreciably overgear the content. Even if making the job harder increases the difficulty from "Very Easy" to just "Easy", that's still more stress you're putting on the healers and by extension the rest of the party.
As for putting stress on the healers, it would be far more stressful if they had to routinely contribute DPS in addition to keeping the raid alive. In my opinion, for end-game content, that's the last resort.
I think most tanks grossly overestimate the importance of VIT past a certain point. Maybe their healers really suck.