https://www.reddit.com/r/ffxiv/comme...tat_intervals/
So every 33 tenacity FROM THE BASE adds 0.1% mitigation, this means 2000 =/= 10% as the base is not 0, and its 330 for 1% not 200.
Having more damage in a large pull from direct hit will potentially make your healers life easier as everything dies quicker meaning your mitigation has more uptime. the 3% or so defence difference between full tenacity melds and just gear tenacity will not make you noticably more tanky, thats just anecdotal. I mass pull all my expert roulletes with my full direct hit melds and I don't die, and I have done since ive played this game, if people rotate cooldowns properly no matter what melds/no melds you have mass pulls are easy.
Edit: if you want further proof see here:
http://theoryjerks.akhmorning.com/stats/ten/
You need over 3500 tenacity to get a 10% mitigation