Collectibles are the MOST efficient way to generate crafting/gathering Materia V, VI, VII, and VIII of your choosing.
The system already functions within the crafter/gatherer endgame UNLESS you don't mind spending millions of gil buying materia from the MB for overmelds.

Collectibles still matter quite a bit to those who want to be frugal or newer players that aren't sitting on a stockpile of gil.