He uses flats when he feels like he needs them - on rougher DH race courses where there's a good chance of needing to unclip and where he knows it would be tough to get clipped back in. But otherwise he still feels better in clipless for basically everything.
I've been stuck in a clipless pedals falling at a standstill trying to get through an uphill obstacle enough that the confidence of flats makes a huge improvement for me.
I think a lot of it is how
comfortable one is with clipless in the first place.
I've had my share of falling sideways from standing still, but it's relatively rare. Beside, it doesn't really hurt falling at that speed. (ok, bruises but I don't consider that "hurt")
For the most part, I just don't feel like I need to unclip in much of what I ride. I just ride... or crash! Needing to put a foot down is a relatively rare situation. So the ease of flat pedal really isn't that strong.
That all changed after I stopped riding off-road for several years (got too busy riding on road). When I came back to riding technical trail, I wasn't nearly as confident nor as proficient in "cleaning" them. So I would hesitate a lot, attempt stuff I used to be able to clean but with the wrong technique or wrong timing... etc. A lot of falls later, I decided I would switch to flats so I could put my foot down when attempting stuff I knew I could but lost the timing/technique.
Basically, "practice" sessions on tougher obstacles are better done on flats.
So far, I'm still riding 90% road and only 10% off-road. So I never got back to as proficient as I used to. Hence just about all my off-road rides are "practice" sessions when it comes to obstacles these days!
The flat pedals stays on. And I think most likely permanently, because I just don't see I have the time to go back to a huge amount of technical off-roads again.