Because the whole premise of a phb2 or three is it will contain important core material, not just supplementary material.
Because the whole premise of a phb2 or three is it will contain important core material, not just supplementary material.
But, I may run the game for a year (or years, or a lifetime) without books 2+. If you didn't tell me those books existed, can I be happy with my game? Yes? Then, those books are supplementary.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.