Mercury treads might be the most overrated item in the game
Mercury treads are bought in games as an anti-mage item when it should only be bought as an anti-cc item.
If you want resistances simply buy a null-magic mantle for 400 gold and get the same MR or a negatron cloack for 850 for 45 MR and become unkillable. I swear if you buy a negatron cloak vs any burst mage they will not be able to play the game
Of course you should only really do this on champions that can complete an item with it but stop buying merc treads just for the resistance, it is 1350 gold. However every champ should at least be able to buy a null-magic mantle as it builds into a lot more items
Even in games where enemy has CC plated steelcaps may be better. If you are to get CC but have plated into comps with auto attackers the plated steelcaps will help you not get killed even if you are CC.
Even in games where enemy has CC usually CC is skill-shot based so you should trust yourself to dodge it or not put yourself in a position to be able to get CC.
I think the only time you should ever buy mercury treads is when the enemy has CC that makes it so you cant play the game and CC chains, such as a leona with a leblanc. the leona will allow a leblanc to land her full combo on you