I think there are two reasons behind the "poke with a single weak girl" mentality. The most important is seed bearer, but personally I saw that type of boos a single time, and it died fast cause some retard did 90% dmg to it.
Another reason is probably that out of RB events, there are times that there is so little bosses and they die so fast, that if you want to get your daily with bosses done, you basically need to use the single girl speedy tactic to poke up to 3 times single boss before he goes away. Only issue with that reason is that it losses it usefulness the moment you learn about multiple ways of refreshing boss list (game restart, unit upgrading or gifting, changing squads, etc.).
Personally I have 230 bells more to use and stopped doing event map cause I don't need more bells, and as long as LCs are nice, I rather try 3 medalling 14-X maps I didn't medal yet, and maybe if get lucky I will be able to 10+1 spin this events gacha in hopes of getting Ivy (she smexy). Only few Manyus left to buy from the shop (4K5 total stars to farm) so I am basically done and ready for next reissue.