They're not quite characters, they're not quite vehicles, but more importantly, right now they're not quite anything in Star Wars Battlefront. A lot has been made of Hoth in the game, though one of the defining aspects from the films hasn't been reproduced, despite the planet itself getting additional themed content in the latest DLC pack. Tauntauns should offer a quiet mode of transport for the rebels, similar to speeder bikes on Endor. They don't need to be insanely powerful or rapid-quick, but they could help you get back into the battle sooner. In modes like Walker Assault - where the Rebellion need all the help they can get - tauntauns would be a great boost for their chances, and would be relatively simple to introduce.