In 4e, I don't like how large a proportion of your defense is measured by your magical masterwork heavy armor. I'm perfectly happy pretending people who are not wearing their heavy armor have a -2 penalty to AC, compared to the AC they would have with their armor. This way, not wearing your +3 plate doesn't give you a -13 penalty to AC, it only gives you a -2. If they don't have time to strap on their shield, that might be another -1 or -2, but they will never have a penalty larger than -4 total, which is still pretty hefty, but not crippling like a -13.
This works nicely for situations where they are attacked while resting, or in water, or in an area where they are not allowed to bring their equipment, or are captured and stripped of their equipment, etc.