If you are playing a classical guitar you'd need to have hands like a gorilla to be able to wrap the thumb round to hold down the bass string - so I suppose it makes sense to keep the thumb right pressing on the back of the neck in the approved classical technique.

The same would go with a folk guitar with a narrower neck, if you have small hands and even so can't reach round the neck.

But if you are able to reach round with the thumb and hold down that bass string, then if you don't feel free to do that, and practice it so that you can do it, you are throwing away a valuable digit if you are just using it as a clamp all the time.

Obviously when you are playing full barre chords, you have to have it in the classical position, but for an F shape I find it far easier to have the thumb fretting the bass string, and the same when I slide up so that it becomes a G or an A or a C chord up the neck. And the same goes for using the thumb in a D chord to give an extra bass note in standard tuning.