I swap back and forth depending on the ride. On Thursday morning I did a lap at my local XC loop - XC bike with clipless. On Friday I rode the enduro bike at Mt Penn - flats. I will generally run clipless if the ride will be on trails I know fairly well and one of my goals is energy conservation. If I think I may be sessioning something or there is a chance I'll be stopping to look at something before riding it, flats are just easier for me. I did that a few times at Mt Penn: "What's this? Oh... Just a steepish roll-in... Let's go!" And I just hop on and roll in, without having to think about clipping in. We also stopped a lot during the ride and were wandering around looking for the trail (it was REALLY leafy and some of the trail marking leaves a bit to be desired...).
My jumping is still better on flats than on clipless, but it's getting better with the latter. I can bunny-hop pretty well with flats, but it's definitely easier with clipless.
I suppose this is a round-about way of saying I really don't understand why folks seem insistent on picking one or the other. I'll run either depending on the situation. Sometimes it just depends on what I feel like riding that day.
