That's just incredibly stupid, and I thought the disposition system was the game's big C&C-hook.
tuluse defend the system to me, or don't if you have something better to do.
How about this: People know you have a reputation for being honest with some people, and being dishonest with other people. By talking in a certain way, you can convince people them that you're currently operating in one mode or the other.
So, a person with a dual reputation might put on his shifty smirking face when saying a line that requires the "Desceptive" disposition, or look deep into the other person's eyes and speak with a soft voice when saying a line that requires the "Honest" disposition.
"Oh, I've heard of this guy, when he talks like that, that means he's being serious."