When you see someone unclip, midair, because they’ve just yanked the bike off the floor with their cleats, then not have the slightest clue what to do about it, before crashing and burning, THEN you’ll know what I mean!
Been there, done that – learned not to! And I learned that before I ever rode flats on MTB.
I set my Mrs riding with flats on her first mtb. As soon as she gained any tech expertise she asked for SPDs, despite having never worn them and despite my warnings that she’d end up falling over a few times at first. And she did fall over and fall halfway down a hillside which shook her up a bit, but she still wants to keep them becuause they help her so much on the climbs.
As for people who say ‘I can out climb my mates on flats when they are on spds’ that means nothing, you’re just a better rider than them. Olly Beckinsale on flats will thrash me on SPDs, however there are things I can’t clean on flats that are fairly easy on SPDs. Likewise I do stuff on the downs on flats that I don’t want to attempt with SPDs.