Philosophers are scientists, scholars, artists, engineers, theologians, musicians, playwrights, authors, mathematicians, craftsmen, and doctors. To limit their capacity to one subject would completely subjugate their spirit.
Public discourse and teaching and consultation are a very common trend among historical philosophers, depending on which specific philosophers you envsion when you think of the term.
I think that they perhaps could be a dwarf that somewhat repeatedly gets fell moods, but there is a strong variable chance that they will never be upset for not honing their trade. Perhaps they could only get a little sad, but something happy could quickly draw their attention away. Perhaps another time, they could just decide to pick a pick and mine out a random shaft with you for 10-30 tiles, and then be gone. Perhaps they could go to fell trees with your woodcutter. Maybe randomly decide to carve some bones.
I would think of a philosopher as someone who would have all non-military jobs turned on, heavily socialize with dwarves they encounter, occasionally get in a rare fell mood and become some legendary something, probably not even get upset that they dont complete their masterpiece thesis. Perhaps a philosopher could even be inclined to train with your warriors to become a fighter, if he so wishes to.
Aristosthenes, Socrates, Plato, Aristotle, Nitsche, Sir Isaac Newton, Alan Turing, Galileo Galilei, Copernicus, Kant, Voltaire, Thomas Paine, Ben Franklin, Nicola Tesla, Homer, Ovid, Einstein, Da Vinci, Monet, Van Gogh.
A diverse group of men and women throughout history that have contributed great things to the world at large were great philosophers. Philosophy is simply raw, unbridled thought and contemplation profounding containing either masterpieces and discoveries or silly and thought-provoking commentary on life.
Perhaps a philosopher is just a dwarf that rolls the dice _way_ more than anyone else.