I have a main breaker in the panel as well. My electrician insisted that I have Interrupt Rating of 22kA vs 10kA on the breakers. He was always long winded on his explanations and most of it went over my head. He said you couldn't mix 10kA breakers with 22kA breakers, something about the weakest link or something to that effect as I grumbled about the difference in price. I never won a argument with him and that is alright with me since IMO he did a bang up job.
The only time I was able to get his goat was when the dry wall guys covered up a couple of his outlets. He pissed and moan for an hour about it. As I was leaving I turned and said, "your measured each outlet position at least three frigging times and you can't remember where the hell you put them!!" then I rushed out the door.