Thomas Shey
Legend
I feel like narrowness isn't an issue in works of fiction, because the space is limitless at every point. Folk seem capable of taking the most narrow premise, and expanding it to fill a complex and fascinating work. And the most open premise can result in something vacuous. It's hard to explain, but I actually feel the narrowness or otherwise of the premises has potentially no interaction with the authenticity of the player's interpretation.
The problem is that there are postures from which people play that are not as constrained as a typical written character. While the latter may be developed in a somewhat freeform way that the writer takes, he's still aiming the character at at least a general sort of end (though he may not be sure exactly what that is until he gets there). People absolutely can play that way, but there are many who don't, and don't want to. A more constrained campaign setup is more likely to leave them in the metagame conflict between playing their character in a way that seems authentic to them or staying in the campaign. This is more likely to be avoided if they and the GM involved were on the same page at the point when the character was created, but it isn't a certainty, and often that "being on the same page" isn't a given.