User blog comment:SlashZero/Winter 2016 E-2 (Hard) Maruyu Leveling/@comment-4713170-20160216063434/@comment-861160-20160217024709

You're right, you don't have to use only one sub. However, the resource efficiency drops if you do, and there's not too many cases where you'll want to grind a sub for exp only besides solo Maruyu. Only the flagship will gain morale.