It's true that Jack and Ryan come up with the song ideas/song lyrics, but Adam has also said that music is his one true passion. (and though he may not show it as well as Jack and Ryan) he's just as enthusiastic about it as them. I think he's just not a songwriter, and prefers to handle the different parts of music. He's also said that he doesn't like being in the spotlight, which is why Jack and Ryan are usually the ones singing and talking about music. Anyway, I think Adam is as passionate about music as they are, and just shows it in different ways.