Lines out of Bill like "What?! Who told you that? Don't listen to him!" are so valuable because it's important to me that Bill is a little bit of a dumbass. Occasionally he's just stupid.
Yes, he's a very successful manipulator, we can see just how easily he preyed on Dipper's and Mabel's wants and fears, and we get secondhand glimpses of just how sinister his manipulation got with Ford.
But also, in real life, many very successful con artists and cult leaders and the like have been kind of stupid, and the reason they succeed is thanks to the average human's instinct to assume that the person they're talking to is mostly telling the truth.
Bill is a good manipulator but this isn't mutually exclusive with him being sort of a moron. And I think any interpretation of him that doesn't remember he's sort of a moron does him a disservice.
