As a repair guy , your repair guy logic makes sense and indeed I do make lots of preparation to files at home either a few days or weeks before a particular gig and of course I don’t delete that work after I’ve done that gig; I keep all those cue points and playlists etc as you never know when they’ll be useful in the future. .
But, as I said, I do lots of additions to files AT the gig. I can load up a track that’s been analysed, so it’s got bpm and grid, but no loops or hot cues and ill think “how can I get this track to mix into the request that the hirer/bride/birthday girl/venue owner has requested?”. Being multi-genre at almost all gigs, almost every mix between any two tracks is a “think on your feet” moment. That might mean quickly applying a loop on the current track to mix out of, and a loop on the incoming track to mix in from - not something which I’ll have been able to plan at home.
In some cases it’s as simple as, during a track that has packed the dance floor, you just add a long long loop or a impromptu hot cue so you can leap back to the beginning of the first chorus every time the beginning of the last chorus comes back round, so that you’re giving them more of what they’re lovin’
Maybe only the DJs and especially multi-genre DJs will relate to this, but whilst a 5 hour gig could be defined as “there’s only time to play 60 tracks, why not just prepare and take those 60 tracks?” the mobile/wedding/multi-genre/open format jocks will know that you’ll probably never know /which/ 60 tracks you’re playing that night, until you’ve played them.
And, we wouldn’t delete those impromptu and adhoc loops and cues from files that night, so read-only drives would be pointless for the reasons outlined