I feel like Defensive Masteries aren't for tanks
The page seems more like a... well... a "special" helmet for fighters and carries that want to get offensive items more safely early game.
Seriously: 20 more armor and 10 more MR while you're in the middle of the enemy team isn't going to do much for you unless you're lacking armor and MR in the first place and the rest of what you do have is health and damage.
15% Tenacity will not do anything for you as a tank. No one will care if you are CC'ed for a shorter amount of time while your abilities are on cooldown - if nothing else you'll always get one rotation off before you die. Alternatively Mercury's Treads can be standard for you anyway.
Nothing about the page says "beefy people get beefier" in a cost-effective fashion. That's what I see.