What I've learned is that, if you've got the bonus units (AND YOU DO, MIKU/MUKU), the random materia/weapons could be useful in the future.
This is basically "Hi, I'm Survival Edges, did you know you'd want +10% HP on a dagger?"
Materia are slightly different. I still slightly regret not getting Lightning's materia. So I totally understand not wasting currency on materia for a character you don't currently have.
My opinion is this: unless you've got a backlog of characters to advance to 6*, go for the unique materia and eq, unless they're clearly 3* trash like Jack or Trey.
A special attack that's useful in an edge case scenario is better than, I don't know, Gigantuars or Rainbow Blooms that you can get literally any other time.