That is true, but those plats he just mentioned are some of the better plats existing so i don't see why he wouldn't use them.
-Gellius is the best tank in the game (well, i guess Deine just dethroned him on DMM).
-Shizuka is the best samurai if you ever are going to use them.
-Thetis is a great valk that is only beat by Clissa (And Lecia, but she is a recent DMM unit so don't expect to see her anytime soon).
-Marnie is the best dancer when you need stats thanks to her SAW.
-Nyashira is great.
-Minerva is the best black when you need high damage per hit.
-Farne is admitedly only slightly better than Solano so if you got Solano during her event Farne is not that important, otherwise she's rather good to have at low cost.

I can't say Cellia is that important to CR though and Sasha might admitedly not need to be CR'd. It's a toss-up whether you want to use rainbow fairies on them or not. Both are great utility but i don't see the need to CR them as opposed to the others.

Of course, i have no idea what other units he has. All in all, if you're not using a unit, don't CR it. If you use it frequently, CR it.