I never took lessons for 16 years.. and the only lessons I have taken since then are from this site.. so I may be just using bad habits.. but..

If my thumb was on the back of the neck, I wouldn't be able to play even a 1/3 of the chords I currently play, the way I like to play them. :) When I look at all the famous guitarists who play electric playing.. hardly any of them are holding the guitar with a classical left-hand grip.. Whether it is more efficient or not, I don't really know..