Armor protects you twice; once because of the damage reduction on hits, once because ranged players tend to prioritize the lightly armored guy next to you.
Might want to add what I remember reading in another part of the forum:
When you are riding a pony it doesnt matter what armor you have, you will get shot, it doesnt matter how far you are nor how many ez targets they have, you WILL get shot, fucking cancer.