Purely cosmetic. The only ones that affect anything are the ones you buy from the factions. They re-route all your earned Crucible and Vanguard rep to the faction you're representing with a mark/bond/cloak. Kinda' silly, yep. It'd be cool if they came with some stats or perks that were helpful to the appropriate class. I have found some that look pretty cool though, so... keep looking. Definitely not the best place to be spending any kind of currency though. :-)