So there has recently been a lot of hate about people running around with their helmets helmets off, and quite frankly I was curious as to why there would be any reason to take off a protective piece of amour in the middle of a battle besides to not break immersion of the game or whatever. So a quick look at this Star Wars Wiki article revealed this about stormtrooper armour:

Its reinforced combat helmet featured an integrated comlink, audio pick-up, two artificial air-supply hoses, and a broadband communications antenna powered by a single power cell. The helmets visual processor assisted the wearer in seeing in darkness, glare, and smoke...

Ok, so all good stuff. But the last sentence finishes with:

though it limited the wearers field of vision.

So I suppose you can always tell yourself that the one running around without their helmet sustained some damage to it (either through thermal detonator or whatever) that knocked out the SINGLE power cell, and rendered the helmet as useful as a pair of horse blinders.

Just a thought.