They look like leather bracers http://en.wikipedia.org/wiki/Bracer_(archery) I don't think there's any issue with fighting shirtless ^^ Alot of Maori fighters fight shirtless so its not unusual to me (And in the case of 300 its MOST welcome XD) I think it all depends of what they're up against.
Yeah- its a movie / DND thing to fight with nothing on XD Classic example- the DND Chain mail bikini. Same armour rating as a ful chain shirt with a third of the coverage ^^; Fail