Fun to hear everyone's stories about their pedals. I've been riding for 25 years and have never clipped in even once so I have no idea what I'm missing. Everyone I ride with clips in, and they are a pretty strong group of riders (I'd say top ten percent in terms of pace up and downhill according to the all-knowing Strava). I came to MTB as a teenager from street BMX and skateboarding, and was initially drawn to dirt jump/freeride/DH so that's probably part of why I never cared to clip in. But at this point I regularly do 2-4000 foot climbs and haven't ever noticed any overall advantage they have over me from clipping in.
My favorite thing about flats? Being able to grab a soda at the gas station after a ride without sounding like I'm walking around in tap shoes!