Yes, I reversed the order of my name because I'm posting from a different computer and I don't have my Ambrosia password at hand.
What I meant was, suppose you're doing a game based on A. Conan Doyle's "The Lost World". There is a group of characters in the story, namely Prof. Challenger, Prof. Summerlee, Sir John Ralston and Edward Molloy (I might have gotten those wrong, but it don't matter). To stay faithful to the original, you want to give the player the option of playing any of those characters. If the player picks Molloy, all the others become NPCs in his party.
I think of RPGs as a kind of storytelling, and stories depend on characters with certain personality traits. If the point of the story is, say, to marry the princess by fulfilling certain tasks, it will be a completely different story if you play as a lowly peasant, a barbarian or an evil wizard. It's a matter of taste, but I like to keep some control over the story content.