Another benefit of ditching the heavy armor is Jump.

Having full plate and tower shield, but without massive strength of a melee class, means you might not be able to hit 40 Jump even with the +30 from an arcane, since your natural jump score will be in the negatives.
I'm addicted to springing around wildly on my wizards, it helps me avoid more damage than any amount of AC (with displacement picking up the slack when I don't jump away in time), and helps with avoiding spells too.