Metatron is an angel in more ways than one! And yes actually, those two are probably the most likely for a threesome, tied with Sarah/Ashley, and liable to be the first I write.
It depends on the area. Some allies have better team support, and some may have special scenes in some areas, ex. Celica gets a big moment in the slaver encounter. None of them are super important though.
I'd probably say that Maya and Aspasia are the most underwhelming party members currently, but not bad with a support build.