Re: Wrapping the thumb over the neck. Why is it considered bad technique?

The following is an extract from a writing submitted to another forum:

"(it is a) misconception that the hand is only operable in its most
primal instinctive mode as a grasping implement, in which to do
ANYthing, its components must contract in direct opposition towards a
central point. This is good for basic tool using, but is not very good
as a one-and-only exclusive basis for left hand technique in guitar
playing, because it is overly conducive of an involvement of the
entirety of the hand into anything to be done. Everything tends
towards subsumation into over-all gesture; chords are dealt with as
though they are objects to be surrounded in ones grip and approached
from outside of their parameters, anything entailing a reach or stretch
is thought of as difficult, and the multi-partite functioning
necessary for handling anything more than rudimentary simulteneity of
movement of musical lines is mitigated against. All this is exacerbated
by a thumb-over position in which the space between the palm and the
neck is nearly or completely obliterated, the fingers compressed and
the third and fourth fingers reduced in function to adjuncts, the
fourth in particular practically relegated to vestigial use. Yet this
is an approach which many never transcend.
With the thumb in mid-neck position and lending counter-support to
the fingers either in direct or INdirect opposition as occasion
demands, and co-support derived from the wrist and arm, the rest of the
hand is freed to be held above and to open up over the strings so that
the fingers are afforded the scope to operate outwardly against the
surface of the fingerboard with equivalancy of access. There are very
few situations in which it would be necessary to disrupt the advantage
of this position by repositioning the thumb over the neck...In either
of the positions described there is a learning process to moving beyond
a contractive mode of operation to an expansive mode, but the (mid-neck
thumb position) much more supportive and inculcative of that mode,
and provides for all the "leverage" necessary for any operation....

That all said, it does depend to some extent on what it you want to do.
You would certainly want to avoid thumb-over for any music
characterized by a multi-voiced texture, but I have observed that rock
guitarists left hand technique, partly because of their focus
primarily on pkaying single linear lines, often bears a closer
resemblance to violin technique that the usage otherwiae employed on