The Phage passive wasn't the only reason Kog'maw bought Trinity Force; he has great synergy with Spellblade, it makes him a bit tankier, and it gives him enough damage and utility to be useful mid-game. Kog'maw also deals a lot of magic damage and has innate armor/MR reduction, so this isn't that big a buff too him, though it is nice.
Personally, I think this item could be a god-send for bruisers since it gives them CDR, sticking potential, health, damage, and armor reduction. Personally, I think this item is going to make Gnar even scarier and help out tanky fighters (i.e. Xin Zhao, Darius, Jarvan) more than ADCs.
Hecarim jungle is also going to love it. Warrior + [New] Black Cleaver + Iceborn Gauntlet gives him a good mix of damage, utility, and tankiness which may bring him back as a popular jungler. I might actually try Stalker's Blade, [New] Black Cleaver, Cinderhulk, and Iceborn when it comes out...