@RogueBurger
The thing is, there are many things that affect what someone might or might not say in a conversation that aren't related in any way to the relation he has with his/her interlocutor. Things like acquired/innate knowledge, wit, the ability to convey a point clearly and efficiently, etc. These are things that, in all likelihood in a RPG, are reflected as numbers on a character sheet. That's the main reason I would want some dialogue options to be tied to these numbers. To enforce some amount of consistency in the storytelling.
Probably, even if everything was open and the game trusted me to pick what makes sense according to me, then I'd do exactly that. When rolling a character in a RPG, I like to decide upon a particular voice, personality and set of values, then stick to that. But then I'd see the game allowing me to be a know-it-all Mary Sue who's always right about everything regardless, and I'd think "wow, that's kind of s**tty interactive storytelling. It doesn't feel like the character I decided to play beforehand makes any difference at all in that aspect of the game".