[@Phoenix] They don't have to per say, but traditionally they do and it hasn't been shown otherwise. The idea is that you have one partner Pokémon and work with wild Pokémon when needed. Or at least, the ones featured in the Ranger games do. Ranger trainers are sorta different from what's depicted.

I remember there being a union in Oblivia so maybe that's a rule set up there...? *Shrugs* I'm more interested in fleshing out Orre than anywhere else. XD