Traps are... rather situational. Aside from a few bosses there isn't much point in using them defensively, they are more for farming elements that you can't buy. You can only purchase offensive elements up to level 4, all the level 5 & 6 attacks and the level 8 summons can only be obtained via trap. (You might be able to grab a couple of the lv5s through chests, I can't remember).