It's not exactly easy to foresee how a dialogue will turn out, and what kind of options will be on the table. Again, I'm not just talking about persuasion / intimidation, but a lot of other dialogue options that depend on what skills you are proficient in.
Also often times, dialogue initiates by itself and just picks a character at random, and then you're stuck with that random choice and your party can't help you in the conversation, which I would find goes very much against the idea of DnD.
