Take "good", for instance. If you have a word like "good", what need is there for a word like "bad"? "Ungood" will do just as well - better, because it's an exact opposite, which the other is not. Or again, if you want a stronger verison of "good", what sense is there in having a whole string of vague useless words like "excellent" and "splendid" and all the rest of them? "Plusgood" covers the meaning, or "doubleplusgood" if you want something stroner still.