First of all, pitching and defense are just as important as offense. Second, we didn't let Bay walk because of his fielding, we let him walk because he wanted too much money and his knees weren't any good. 3rd, we didn't replace Bay with just Cameron, we replaced him with Beltre and Lackey too. I think having those 3 guys is more valuable than having just Bay.
Finally, sabermetrics aren't perect, no one is arguing that they are. Defense is important, but it's just a piece of the puzzle. It's about balance and value. There isn't anyone on our team that can't hit. As for Randy Winn, yeah, I'd take dunn over him too, but you are talking about a .1 difference in WAR. It's insignificant and hardly a great example if that's the best "gap" the guy could come up with.
It's not even that difficult to understand really, someone who is faster has more range and will get to more balls meaning less runs for opposing teams. Dunn is a little leaguer on defense compared to Winn. Winn's much faster, has a much greater range, much better glove, anticipation, can dive, etc etc. It could very well be the difference between winning a game 2-1 and losing 4-2. Now, i do believe Dunn's bat more than makes up for it, but I'm not actually all the surprised the value is so close.
I do want to point out that having good pitching is vital to defense. If you can't pitch there is really no point trying to build a strong defense. I know it's obvious, but pitching and defense go hand in hand, weak one and the other will become weak as a result, and vice versa.