When training for emergencies the aim is for them to learn a process which will work time and time again.
If you teach them that they can glide further by stopping the prop, which may or may not work, you engrain in them an action which will not work when things are most likely to go wrong. The EFATO scenario.
Yup, no doubt if you have time and altitude you can worry about gliding a bit better if you stop the prop.
The marginal increase in performance which can be achieved does not outweigh the risk of making this issue even more complicated.
Let's face it, how many times have you been impressed by someone doing an unexpected PFL?
FD