Hmm, thought-provoking. I do agree that by default interactions should be with your main character if they're around, but also like the idea that you can send in a party member for some conversations they may be better suited to either because of higher charisma or other factors such as race or their story. Plus your main character may be elsewhere, sneaking, or temporarily dead or unconscious.
And given I do want other party members to be able to lead all or part of conversations, I don't want to have no influence over what they say if they do. Personally, as long as the game gives me at least one line that it's feasible the companion would say, I'm okay to take responsibility for not picking wildly out-of-character lines for them. But I agree it would be better to remove some options that completely go against who they are.
