If you are only getting one rear wheel, and don't want to deal with a cover, getting a disc makes sense. The main disadvantage is convenience in day to day use, e.g., need to use a pump adapter, possibly different valve stem length from your front wheel, and a little more of a pain to transport (I try not to put a disc on my hitch rack). You could also get the random race director who decides last minute to ban discs, though that has yet to happen to me.