Steiner gives the most half-assed reasons for going back with Beatrix and Freya. He also leaves Garnet. the princess he is sown to protect, completely in the hands of a person he has vehemently disliked most of the game.

And Quina comes out of the water in the Eiko's Kitchen ATEs. He's then just standing there with one of the moogles for the rest of Disk 2. There's no reason s/he couldn't come with to the Iifa Tree either time.