I doubt that I'd install a main in the sub panel since it is within the same building. Still, those panel packages that come with a main PLUS a selection of breakers are sometimes less expensive than buying a MLO panel and the same breakers that come with the package. Kind of hard to argue at that point.
A 100 amp MB panel with five SP 20 amp breakers (Square D Homeline) is only $60.00. In that case, the customers are getting a main breaker whether they need it or not.