That happened to me a lot. This drummer never practised with a metronome, and I did a lot. Everytime he slowed down, sped up, played a polyrhythm unevenly, etc. I caught him. He would always disagree. "I'm a drummer, I JUST CAN keep time!" Doesn't work that way.
Anyway, this brings me to another point. Playing things like triplets or songs in funny time signatures can be tricky when just using the nod of your head and tap of your foot. Metranomes assist geatly here, so that you dont carry a triplet over for 1 and a half beats, or speed it up, or something. (Just something that drummer did again)