I never unclip the left foot prematurely. You'd be surprised how quickly you can unclip that left foot when you realize that you're about to hit that barrier at 15+ mph. That being said, I have personally crashed into, and taken out a triple barrier (feeble pvc pipe) during a race because I could not unclip in time, which I attribute to a lack of attention and bad timing.
I think you're much more stable with your left foot clipped in -- especially if the course is slick. Clipless pedals, metal cleats, stiff soles -- not a lot of friction holding your foot on that pedal. Plus, if the course is bumpy (and when isn't it?) you could easily be knocked off that left pedal while swinging your right leg over.
It just doesn't seem like a good idea.