Thanks for the input.
The main reasoning for this advice is to begin with good habits. If a student is allowed to dive in a limited way from early lessons this may be embedded (and very hard to remove) from their resulting flying style. If they meet a situation in the future that COULD be avoided with a dive then they might just reach for that 'tool' and do it much more quickly or agressively depending on the urgency at the time. Disaster awaits.
Of course, a limited 'dive' can be made, we do that every time we transition away or adopt an accelerative attitude if you wish to see it that way, and in types with different head construction/design a much greater dive is a perfectly acceptable maonevure but for students beginning (and maybe never flying another type in some cases) I decided to stick to this, the safest option.
It is probably due to the way I approach this sort of thing when teaching, ie. with negative G. teach what you must not do first (giving the strongest basic pattern or 'habit')and learn later if that can be modified (if necessary).
If no-one ever 'dives' then Negative G is much less of a risk? Just my view....