I've seen people do this on some of the STW rides, especially on steeper stuff. It always looks bloody lethal to me, how are you supposed to get weight back, balance, put in pedal strokes etc if you've got one foot disconnected.

IMO, and you're welcome to ignore it, you'd be better off with flats if there's much stuff that technical round you. Sure, you'd be slower on the climbs and on the flat. But it's not a race. Also the techy bits are the most fun, so any compromise should allow you to get maximum enjoyment out of those sections of the ride.