User blog comment:Obyvvvatel/Magic resistance isn't effective./@comment-6061249-20130516210047/@comment-5266525-20130516231732

Your calculations would be correct IF you were not assuming damage outputs for physical and magic damage are the same. They're not.

Early game, AD casters with massive ratios let them brute force their way through armor while higher starting MR values mostly keep mages at bay. Midgame is where MR is less useful, which is why most mages have their strength in the midgame. High armor by this point already takes AD casters out of the picture and AD carries haven't started revving up yet. Then lategame the sheer amount of MR a Bulwark gives is enough to reduce most mages' influence and a full built AD carry doesn't care about your 200 armor.

Making MR any cheaper on top of the 30 starting MR will only end up driving magic damage dealers off the cliff even sooner. Maybe they wouldn't even get up there in the first place.