2% is pretty big.
For context - Everyone in a party gains an extra 1% main stat gain for having one of each role in a party - Tank, Healer, Ranged, Caster, Melee. If you took 8 of the best players in the game, that 5% difference (1% for each role) would mean about 30-40 seconds in kill time.
So...2%, on a tank, sure, its not going to prevent you from clearing content, but it is simply silly to meld something sub optimal when you KNOW that tenacity is by and large a worse stat to meld and to priorities.
At the end of the day, Vitality, and by extension general ilvl is whats going to get you your biggest eHP increase. The difference between lvl 80 AF gear (ilvl 430) and raid gear (ilvl 470), 40 ilevels, is about...30k-40k flat health...However, the difference between ZERO tenacity and have as much as you can possibly have in game right now? It doesn't even breach 4 digits in terms of eHP as far as im aware.
So to summarise, having an extra few hundred eHP will not prevent a healer to have to heal you, the whole point of mitigation, and so it's just flat out better to take that 2% especially when you consider both grade 8 tenacity, and grade 8 dh/crit is bought for the same token, Cracked Stellaclusters...Not like there is much else to spend gil on in the game anyway, so even if that wasnt in the game it still wouldnt matter. If, for example, Tenacity allowed the healer to have to heal you a noticable ammount less, it'd be worth it, but it simply isn't.
It's also worth noting that tanks have 100% uptime on the boss, or atleast as much as is possible in any given fight. Healers, meanwhile, are casters and have to aoe heal the group and thus dont have 100% uptime. a 2% increase on tanks is simply vastly more valuable than even an equal 2% on healers, would that tenacity provide that.
....Also, Why is this thread in the General discussion, and not the tank roles subforum? Someone light thebatmod signal.