I don't think dragging the knee is the goal, it's just the result of carrying a ton of corner speed and lean angle. The rider shifts his weight (hangs off) to aid in turning. The knee goes out for a little stability and to possibly catch a front end that's washing out. If you look at professional racers, they're not trying nearly as hard to put the knee down as some of the weekend warriors on the street.
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Even Ago and those guys shifted their weight around - they only "sit" on the seat on a long straightaway. They carry their weight through the balls of their feet on the pegs so they can slide forwards, backwards, side-to-side. Most of us do this without realizing it - that's why your legs are so sore after a good ride.
Anyway, my answer is "sometimes". I live near a big school parking lot that serves as my test track on the weekends (if nobody's around). I've got some good 20-30 mph turns laid out that are perfect for the 400. I definitely get around faster when I hang a cheek off the seat, but putting the knee out doesn't really help at those speeds. I don't have anywhere near the amount of skill to push the front or slide the rear tire in a controlled manner, so I doubt I ever will do it.
These guys make me want to try though:
[youtube=425,350]iVExD-Fz688&rel=1[/youtube]