Not sure how accurate this is. I've seen parry come up on a number of magical attacks, and based on what I've seen from other posters, it seems to affect both physical AND magic. Evidence is purely anecdotal, though.
Even so, isn't it still more valuable for a tank to stack anything that will reduce incoming damage? Even if only by a little bit? Sure, encounters right now don't really make mitigation valuable (because of how little damage is going out), but as a general rule, isn't anything you can do to even have a chance of slightly reducing incoming damage worthwhile?
But what about trash pulls, or bosses that have frequent auto-attacks? You're listing one example, but that example is why cooldowns exist. Parry seems more to exist for helping to just reduce incoming damage in general; as a tank, isn't any little bit of help we can give to our healers worthwhile?
This is the only point I can kind of understand--but even so, wouldn't it still make sense for the MT to equip Parry to smooth incoming white damage? Especially when DPS is only an incidental goal of tanking, doesn't it make more sense for a Main Tank to do whatever is within their power to have the greatest chance at reducing incoming damage as possible? DPS secondaries are also not useful for period of outgoing damage where the boss cannot be attacked, while Parry is/might be.
Not trying to argue, just really trying to wrap my head around this, because it seems completely backwards to me.