I received a copy of a home inspection report on a brand new house we just wired. I've never heard of it either. The report states: "Arc fault breakers "stacked" in the panel - they should be separated by typical breakers"
Then again, another "problem" found was: "Master bedroom outlets at bed location on arc faults"
The house is big money, a little south of 3 million, and I assume the report is being used as a negotiating tool.
I do wonder what happens when you get 15 or 20 AFCIs in a panel that is already living in a un-airconditioned garage that cruises in the 35c ambient range anyway. Breakers are usually rated at 40c (105f) or less. I guess this will be like all the other rules they pass with unintended consequences. It does sell books, classes and CDs. I think that is the main function of NFPA these days.
It seems obvious to me that the heat situation pertaining to AFCI's is the reason Eaton submitted the proposal to end the 42 max OCPD's (which has been accepted in principal) in a panel, this will go along with their proposal for all circuits to be AFCI protected.
It will allow more spaces for selective positioning of the AFCI's