Pondering rhetorical devices such as "with all due respect"

One character, Richie, appears on the scene after a long stretch in prison. Richie is not pleased with the changes that have occurred while he was out of circulation, and is vocal and aggressive in his criticism of the decisions and leadership of the current crime boss (Tony). It seems obvious that Richie and his boss are destined for a showdown, but in a plot twist Tony is delighted when Richie ends up dying at the hands of his lover after a domestic quarrel.

The way in which Richie expresses his dissent is interesting. He always prefaces it with the phrase "with all due respect." Then, in words and tone that are totally devoid of respect, he challenges the boss and goes on to disobey his instructions.

'That's a very good question'

This made me think about the phrases we use in medicine. It is rare these days for me to attend a lecture and not hear (during the question-and-answer session after the talk) the speaker fail to respond to a question with the phrase "that's a very good question."

"What does it mean," I asked a friend, "when the speaker responds to the third or fourth question with 'that's a very good question'?"

Does it mean that the other questions were not good questions, so the lecturer is finally happy to get a good one?

Does it mean that the speaker doesn't know how to answer it, and is playing for time to think up a response?