Sense is much more useful if it is your first RPG, as it was for many people. If you played the previous titles, or are replaying the game for the millionth time you have a fairly good idea what is weak to what.

I actually use cover quite a bit. I usually go with Cover + Hyper status, so that all the damage goes to the character with the best limit breaks.

Now in terms of useless I throw HP-MP Switch materia in the ring. (Switch? Flip? I don't even remember).