True... he didn't have on any armor after he went through his first transformation, so I think he just wears it just for the sake of wearing it.
Oh, maybe this reason, too: he wears it to make himself appear weaker, and thus give him the upperhand when dealing with a foe. If they think he is some small little weakling who NEEDS to wear armor, they will be less likely to be on guard around him. I mean, look at Vegeta. He was always talking smack about Freiza, yet he got whomped by him when they actually fought. It's a tactical piece of equipment.