Nope, they are not junk items. I find them useful for my lower classes and I simply put them in lower armours to sell to add a plus value to it (sells faster). Also, forbidden materia allows you to put many lower tier materia in your gear to mimic higher tier materia (it is risky, but still easier to get many lower tier materia).

If you won't use lower tier materia, why not give them away as presents to new players?