Socrates (AKA Plato) was a badass philosopher who would use the Socratic method
of logic and reasoning to get people to prove themselves wrong!
He would never directly tell you you were wrong, but instead pretend to be very interested in your point of view, and then ask you a series of loaded questions until eventually you would be arguing against your original statement. Read The Republic or The Symposium if you have the chance.
Anyway, I agree that the philosopher unit must be slow, weak and weaponless, because they were typically pacifists. (Unless you make a Philosopher King hero, which sounds awesome already). My version of the philosopher would roll against other units that get within his attack range. If he wins the roll, he has successfully convinced the other unit that your cause is righteous. That unit will then come under your command until it is killed or re-converted by the other team. (Philosophers cannot be converted)
Socrates always used to believe that only philosophers should be kings, so a Philosopher King hero would probably also get to wear armor and carry a sword.