Your question commits the Manichean fallacy (either/or) – we don’t have to decide between these two alternatives. A piece of drama can be both entertaining and instructive, and the best drama does both. Waiting for Godot is as good an example as any – on stage this play is utterly mesmerizing, life-changing really. Does it teach a lesson? Undoubtedly; several in fact. But was it Beckett’s intention to preach, or even to change people’s minds? Certainly not – anyone who knows Beckett knows that his intentions are as far from lecturing as they are from commercialism. Modern playwrights, the artists anyway, are trying to give their plays lasting literary and dramatic value; much of that value dwells in presenting a worldview. The “lesson” of Godot is that we (including humanity itself) are all waiting, for meaning, for purpose, for direction. And finally, very seldom can we blithely assign artistic motive from artistic product. Entertainment is too broad a term to bear much weight in discussing live performance.
Tell that to my professor! Agreed.