Offense and defense are very different. You can only make 3 positive contributions (outs) on defense per half inning, but on offense, you can continue, theoretically, infinitely, provided you don't make 3 outs. There's no limit, so you want as many good hitters as possible. Hope I'm explaining this well, it's really JFaS' point and he originally explained it far better than I have.
As well, it's not really about trade offs; baseball is a zero-sum game. Some players' s***** defense outweighs the fantastic offense, and vice versa. You don't just tolerate one side of the coin for the other; you look at it as a lump-sum, and it is what it is.