I can’t think of a better description of why I teach registers the way I do, following the precepts of so many of the old school teachers, than this (rather lengthy) quote from Husler and Rodd-Marling.
In a proverbial nutshell, this is why I do what I do:
Another ‘superstition’ that should be mentioned concerns the current belief that ‘registers’ should never be practised separately because it leads to a so-called ‘register divergency’, i.e. ‘an unhealthy separation of the registers’ which will eventually prevent them from blending.
A reminder: the optimal, the physiologically correct tone, is produced by the mutual, unified functioning of a whole series of musculatures. But how can several functions be practice simultaneously when possibly each one is working badly, i.e., because the lack of freedom of one musculature is constantly hampering the others? Here we have no choice. Muscles have to be practised separately until each one is able to function cleanly, when nothing will obstruct their joint action, nothing prevent them from ‘blending’. (Manuel Garcia clearly expresses his opinion on this subject: “To overcome the material difficulties of his art, the singer must be able to control every part of the mechanism to such a degree that he can use the functions separately or together as required.’) It must be stressed that, at first, this procedure is not entirely without danger. One has to be certain not to provoke any chronic over-accentuation of one function, of one ‘register’ (we shall be referring to this in a moment) which would lead eventually to an unhealthy splitting of the voice and the serious consequences involved.
Individual functions in the vocal organ as needed in singing have to be roused by being exercised alternately. Practising one function too long easily leads to over-training and generates trouble of a different kind. Immoderate use of one function reduces the activity of others with which they should co-operate. This, in turn, disturbs the functional equilibrium of the forces at work. Muscles that are chronically over-emphasized eventually lose the capacity to amalgamate. Such a disruption of the functional unity is present in every normal vocal organ, and to a lesser degree (though they may sing quite well in spite of it), in that of every professional singer.
We see therefore that functions must be practised alternately, but that the time spent in exercising each one must be scrupulously apportioned. (Careful dosage has nothing to do with ‘sparing’ the voice; it means how long each function is exercised.).
Husler, Frederick, and Yvonne Rodd-Marling. Singing: The physical nature of the vocal organ: A guide to the unlocking of the singing voice. Hutchinson, 1976.