Example: A discussion about movies………..someone says “I’m sorry, but I don’t think Brad Pitt is a good actor.” Well, that’s fine; You have your opinions. But the “I’m sorry” isn’t necessary’ it doesn’t even make sense. Just say “I don’t think Brad Pitt is a good actor.”
I'm sorry, but it means "I'm sorry to disagree with you, although we normally are of one accord".
No, actually in this example and others I could cite the "I'm sorry, but" often comes before anyone has expressed an opinion. In the Brad Pitt example, the person was the first to speak, and I never said what I thought about the movie or his acting. For all they knew maybe I detest Brad Pitt, so the "I'm sorry" is presumptuous and pointless.
And, I don't feel it's necessary to apologize for disagreeing with someone - especially when the discussion is about something so innocuous as a movie/actor's talent. As far as normally being in accord, that's not trur either because I had just met the person and haven't seen the person since.
While I realize your assertion might apply in some cases, it's not always applicable.