Honestly, the last couple floors in the main game are unbearably annyoing without Chen + Rumia/Renko, especially since you can easily get there while being 10-20 levels "behind". Enemies casting Ether Flare, Calming Scent or Black Universe while being ridiculously fast are just a pain, especially if you're using Strategist for first turn instead of Aya. Several of the "extra" postgame areas are in the same boat, honestly. Even after that, I found Rumia quite useful for not shooting past the challenge levels in Plus Disk so things can be actually challenging without special effort.
Also, one thing that deadweight backliners can do is power up anyone with
. That subclasse's buff absorbing skill works on allies in the back as well. If your back liners have buffs that activate even in the back[Sorcerer/Warror/Enduring Celestial], then your buff absorber will get a head start from them. This works even better with Ran's all buff skills, to the point where they can give the
character 100% buffs in the relevant stat from 1-2 casts. The negative of this is that they'll also leech away the buffs of Byakuren or your squishy and hard to buff attackers.