Do I really need to keep CPI file from AVCHD container?
None of the editing tools I know of uses any files other than MTS/M2TS. Supposedly, these CPI files can be used by a Blu-ray player when it plays the content of a card - which is why the structure of AVCHD media resembles the structure of Blu-ray media - but I never play raw videos from my cards.

I preserve MTS/M2TS files only and throw away everything else. Not once in twelve years I needed them.