Yep, couple of my mates do, I have on occasion but it depends on the type of track and I have a pretty loose cornering style, I get a fair bit of slip going so being clipped in freaks me a bit, I'm also pretty good at pedalling over rough ground on platforms but did notice the improvement with clipless.. Alot of the WC downhill racers do but it is all dependant on style. The main advantage when it comes to DH riders is being able to pedal over rougher terrain and not get as bucked around on rutted parts of tracks/rockgardens.

It is a strange feeling to me, I prefer my clipless pedals on my roadie and platforms on my DH bike. Now 4X and XC, that would be a different story, I would wear clipless on an XC ride and most 4X tracks.