If i may, watching Moto3 gave me an idea of when it is useful to get your knee down and when you don't.They can't afford to stick their knee out for no reason as they would lose too much corner speed.Only on certain corners, they do and on some their legs are tucked right in despite carrying a " Knee Drag " worthy lean.
The use of getting your knee down is best demonstrated by Lorenzo IMO [ The cleanest shall we say, almost all GP riders do this ] He will have his knee out all the way even as he is just tipping in and say at 30 degrees lean.Then, His leg FOLDS inward as the bike leans fully and his knee will be tucked right under his elbow.
One variant can be observed at 30 sec ish and another at 40 sec ish in this video.
http://www.youtube.com/watch?v=gPV5Lg9IN7Y