Etherimp, are you correct, no dodge my derp. I meant the auto attack reduction. Tenacity will not stack, but it will stack multiplicatively with crowd control reduction effects. So you get 35 from mercs and 40% from contaminate(rank5) = 61%. A team with strong auto attackers even if its only 3 from the sources of damage you'll mostly be taking is physical. They would have to be really AP heavy or CC heavy to make mercs on trundle better imo. you could easily make up the lost MR by getting a wits end as I do. Best way to do it is to counter build the enemy team.
Show me a team that lacks CC and I'll show you a team that is going to lose no matter what you buy.
Most teams have, at the very least, several slows with a few stuns thrown in. The reason I feel Tenacity is more important than Armor/AA reduction is because Trundle is melee and depends upon sticking on an opponent in order to keep their AD reduced and keep them slowed with Phage procs and/or Red Buff. I believe you also get additional Tenacity somewhere in the Defense tree. Another reason I prefer 9/21/0 over 21/9/0.
I recently got stunned by Taric and the stun didn't last any longer than 1 second. It felt like I broke out of it immediately, because I had Merc Treads + Contaminated ground up. It was impossible for him to peel me off his Ezreal.
10% AA reduction just doesn't seem justified when you reduce enemy AD with Q and steal their defenses anyway, with your ulti.
That said, most guides give you an option between Merc Treads and Ninja Tabi because BOTH are good boots.. I just personally feel the Tenacity is too good to pass up. Of course if I were up against a primarily Auto-attack based team with little CC, I would go Tabi instead.