It's not what the loops are for, and of course some kilts don't even have them. I generally will wear a belt if I have a shirt tucked in; if not, I don't. If you don't have something covering the top of the kilt where the belt goes, it just looks bad to not have one on, IMO; sort of looks like you're wearing a plaid towel. Whether I have a belt on or not, I always wear a sporran.