why would I want to use worm power to command the gnoll queen?
Ever heard about "roleplaying"? Its even in the game genre name, RPG > Role-playing-game ...
So that would be one reason.
Different outcome, since it would be quite boring to replay game exactly the same way imho. O_o
And finaly, to get your XP ... since, as far as i know, if you do, you get XP for killing her, and some bonus XP on top of that.

she will offer you a quest reward, but you get none of those reward if you simply kill her.
Agreed ...
Items should not materialize from thin air, we should have options to steal them, loot them, or get them as rewards.