I'm reading Plutarch's life of Lucullus, right now. How much of a chance did Mithridates have against the Romans? Would he have had a better chance if he had started fighting a few years previously, when Rome was still in the midst of the Social War?
The Italians did ask Mithridates to join the war, they even minted a coin to commemorate his participation. For some reason my google-fu is failing, so I can't find an illustration.
I don't have a clue why Mithridates didn't join the war. I will only say Mithridates thought he had a chance, and in fact survived three wars with Rome and ended up disappearing.