Brilliant idea as too many drivers (and they are not all new drivers) have no idea how motorways work or how fast the traffic will be going, they have no clue how to give more space than normal, how to signal, overtake, pull back in, leave slip roads etc etc
And as it will be down to the instructor who has these lessons it should not present any problem as the learner should have shown enough competence beforehand.
Personally I think motorway lessons should be compulsory before or after the test otherwise some will opt out and these will be the ones who go on to cause the problems
The driving test itself still barely covers the basics and puts too many drivers on the road with little or no experience of "proper" traffic and we all see the results every day.
Done properly and with proper supervision and control this should hopefully help us all stay safe.
I should add my daughter is a driving instructor and is most definitely in favour of it, but like me thinks it should be compulsory