There can be several ways to get enjoyement out of a roleplaying game:

• The sheer intellectual challenge of the game (which you can also get from storyless boardgames)

• Telling or enjoying an interesting story, with interesting situations

• Escapism - living as someone else, in a different world

These are usually called Gamist, narrativist, Simulationist.

They are not mutually incompatible, and you can indeed have different people around the same table with different tastes / goals. There can be problem when one ruins the story or the believability in order to get a game advantage, and other players were caring about the story etc. - this is when people claim about munchkinry.

But you can still have good game sessions where everybody is a munchkin, or when the rules and DM are good enough so that the player's don't get to choose between a game advantage and an interesting story (for example, I think in most versions of D&D you basically have some points you can only spend on combat-useful stuff (opicking feats or powers), and some points you can only spend on combat-useless stuff (skill points)).