using spells and their usefulness on the map and in battle.

Everything is relative, and if all spells were only beneficial, and you could just randomly select one and still get a positive effect, the game wouldn't be as interesting. You'd have to use Killer, maybe Luck, but only if there aren't any more useful spells available (or they've all already been cast). But usually, by the time you get to those spells, the battle is already over.