Many of the characters join the party upon their introduction without any clear indication or motivation for doing so. Many of the plantoid characters are examples of this: NeoFio, Turnip, and Funguy.
I can see this as a point for the characters that you come across in the course of the story like Macha and Steena, but maybe over a dozen characters had no relevance to the story aside from being a party member.
I would have enjoyed this game a lot more if only the story relevant characters like Glenn and Harle joined the party. This would have helped characterization and interactions in the story. I think I saw a list that someone created on another forum several years ago that listed about a dozen characters that would join and leave the party at certain intervals and the story would have been a lot more cohesive if this was the case; there would be no pointless sidequests recruiting plants that sprout out of the ground or anything like that.