I'm with you on that point. But I don't think forcing all player interactions to be through the main character is a solution (and, adjacently, I don't think allowing player interactions to be through other party members is a symptom or cause of the problem you mention).
So what's your suggestion?
I've already offered my suggestion: do it the way every other game does it and have the companion characters speak their lines without player input, when appropriate. But that's no reason for me to be unable to take one of the characters to the other side of the map and have a generic interaction with a merchant.
