I find it interesting to look at the idea of 7 densities in correspondence with the following quote by HPB (emphasis added):
There were portions of the Secret Science that for incalculable ages had to remain concealed from the profane gaze. But this was because to impart to the unprepared multitude secrets of such tremendous importance, was equivalent to giving a child a lighted candle in a powder magazine.
The answer to a question which has frequently arisen in the minds of students, when meeting with statements such as this, may be outlined here.
"We can understand," they say, "the necessity for concealing from the herd such secrets as the Vril, or the rock-destroying force, discovered by J. W. Keely, of Philadelphia, but we cannot understand how any danger could arise from the revelation of such a purely philosophic doctrine, as, e.g., the evolution of the planetary chains."
The danger was this: Doctrines such as the planetary chain, or the seven races, at once give a clue to the seven-fold nature of man, for each principle is correlated to a plane, a planet, and a race; and the human principles are, on every plane, correlated to seven-fold occult forces -- those of the higher planes being of tremendous power. So that any septenary division at once gives a clue to tremendous occult powers, the abuse of which would cause incalculable evil to humanity. A clue, which is, perhaps, no clue to the present generation -- especially the Westerns -- protected as they are by their very blindness and ignorant materialistic disbelief in the occult; but a clue which would, nevertheless, have been very real in the early centuries of the Christian era, to people fully convinced of the reality of occultism, and entering a cycle of degradation, which made them rife for abuse of occult powers and sorcery of the worst description.
-- H. P. Blavatsky, "The Secret Doctrine," introductory.