Partners are moral support in Battle and the last piece of flair you can add to your screen after making the world's coolest avatar.
Their primary purpose (I didn't learn this until after buying one) is to tutor you on what almost every button does in the game as you scroll over them, and to give you very very elementary advice for card duels. IF you find you are lost in the alteil console or find the card game itself confusing then the advice might be helpful to you, but even then maybe not. It's stuff like (Don't let your unit die! You can revive units!) or (This Button Takes You to the Arenas where you can fight Iczers!) By the time you can afford one with FM you should have reached a level where the advice will probably be useless to you. If you are a spring chicken buying one with Gran could be plausible.
Otherwise its a purely aesthetic choice. I bought the magician soldier "Rogue" first... total meathead.
Jimmy Raynor uses Falkow. Jimmy Raynor doesn't lose.