>RPG stands for role playing game. Its about playing a role in the world, alighment helps to cement that role.
Only if you play one dimensional character, if you would like character have more depth it's a detriment.
Edit: also lack of alignment doesn't prevent you playing a single minded character.
>In a fantasy world, morality is absolute.
That's not a rule, there is huge number of fantasy worlds with a more realistic version of morality, for example - game of thrones. Fantasy world having black/white type of morality usually speaks to the lack of skill and imagination of the writer, not some kind of limitation of fantasy genre.