Why is there no "default" magic resist item?
For armor, even when Deadmans is not useful (already high mobility) or sunfire (on ranged but tankier champions), Randuins is always going to be useful because every ADC builds crit. But MR is not the same way. Mask is only really good on champs who do magic damage, visage is only really good for self healers, and helm is only good against DPS mages like swain and ryze.
Make MR great again
