I would most certainly use the 1RM on a beginner just for the motivational and positive psychological effect it would have on that "new" trainee.
The psychology of it is just why I
wouldn't use it. There are three reasons for this. But first, you've said where you're seeing things from: from the perspective of professional coaches. Let me explain where
I'm seeing things from. I should have mentioned this before, I apologise for being unclear.
My perspective is that of an instructor in a mainstream gym, which is where most people train, and where I'll most likely be first employed. This combines with my philosophy that
the best workout is the one you stick to.
You're speaking about the perspective of
professional coaches. But most trainees
don't have professional coaches.
Someone gives them a workout programme and then sends them off to do it on their own. There are things which work
very well when a professional coach is supervising every rep; those things don't work so well when the trainee is largely on their own.
My focus here is on getting people through the first three months; many studies and much experience shows that it takes about 90 days to establish a new habit in a person's lifestyle, whether a good habit like physical training or a bad habit like smoking. If they last 90 days, they've a very good chance of lasting
years.
90% get discouraged in those first 90 days and quit. Some people have noted that "standard business practice is to fail in the first 18 months." Well, standard weight training practice is to fail in the first 3 months.
So we need to adjust our approach so that the person can have steady progress, which encourages them to continue even without anyone there to push or advise them.
Okay, so that's where I'm coming from. Now the
three reasons might make more sense.
The
first is that a beginner's performance - in lifting, in skills in a job, in school, whatever - is
inconsistent. They may lift 40kg today but only be able to manage 35kg tomorrow. That's because of what you've mentioned, that in the beginning they're not using their muscles to their full potential, the adaptations are neural rather than muscular. So their performance is very up and down. If we push them to their limits today, if they don't manage it tomorrow they'll become discouraged.
Now, if we see them make 10x 30kg easily, we know that next time they can do at least 8x 32.5kg easily, if not 10. If we stay within their limits, they'll see steady progress, which will encourage them.
Developing further this "better steady progress" idea we come to the
second which is that - again because of the neural rather than muscular adaptation happening - trainees' performances can leap forward
very quickly... and then stop for a bit. Perhaps someone can bench 20kg in their first week, and then in week 4 they do 10x 45kg easily, they could do as much as 1x 55kg... but then not add a single kilogram or rep until week 8. Wouldn't it be better to have them adding 2.5kg each week, than have them add 10kg and then nothing for four weeks? Which would encourage them more?
Lastly, to
discover their 1RM, we must at some time have them train to failure. In discovering that they can make 57.5kg they may also fail at 60kg. Now, some people will focus on the success at 57.5kg, but other people may focus on the failure at 60kg. Who will focus on the failure? People who have a history of poor performance in physical pursuits - that is, the people who
most need training.
This comes back to what you've so often said about not training to failure, always leaving "one in the tank." It's very common for personal trainers to push their clients to failure on sets. So the client finishes the session remembering that they failed on several sets, and is discouraged.
Again, if Fadi or Markos or (from my PT school) Soviet Boxer were there to supervise every rep, then there are some fantastic techniques which would work well. These are techniques I hope to use if I become a personal trainer - as opposed to a simple gym instructor. But as a gym instructor in a mainstream gym, I'll be limited in what I can do for each trainee. So I have to try an approach which will let them have steady and small progress, to encourage them to keep it up, because the best workout is the one you stick to.