There is a practical reason for him to wear a costume. So we know what he looks like. Quick, what does Luke Cage look like? Yeah, I thought so. Also, costumes are cool and the everyman Luke fights for (by the way, aren't all heroes fighting for the everyman? If Luke doesn't like costumes just have him admit it) like them. Will he get kicked out of The Fraternity Of Street Level Heroes if he looks interesting? If so how does Daredevil get a pass? Iron Fist isn't really street level.