Why does tenacity stack multiplicatively?
Adding to my many posts about this champion and why Dr.Mundo he's one of the worst champions in the game, I've just found another reason for it. Tenacity stacks multiplicatively: http://leagueoflegends.wikia.com/wiki/Named_item_effect#Tenacity
Meaning that Mercury Threads or Elixir of Iron are having far less efficiency (because Mundo's W grants tenacity too). The only champion that relies on tenacity as much is Dr.Mundo, I really don't see a reason for tenacity not to stack additively. Then he finally might get some value out of his kit and move from being one of the worst top laners in the game (50th by win rate) despite being suited for that role.
. He passive+mercury+iron elixir game her 80%CC reduction, combined with her already high tankiness build and her somewhat reliable mobility and damage made her way too broken. Since most champion don't have buidl in tenacity they changed to stack multiplicative so it woudl only affect her greatly