I can understand why anchoring would be a handicap too. You're stuck to one spot across the picking area. If you weren't anchored, you would have full, free motion with your hand and 3 extra fingers to do some fancy finger picking.
^ nothings a 100% ANYTHING. 9/10 times you'll hear "THUMB BEHIND THE NECK, BITCH", and not "THUMB BEHIND THE NECK EXCEPT FOR BENDS, BITCH!". People like to miss the odd case. Still, when not "floating", one should remain "unanchored".
I know you were using it as an example, but you don't even really have to put your thumb behind the neck at all. Jimmy Page plays fine without doing that.
^ yeh, buy he does a fair few bends and has a pentatonic based soloing idiom. You wouldnt want to be doing stretch licks or 4nps scales like that...