This is a very interesting thread. Too bad I didn't have time for forums while my family and I moved across the country. Anyway, as Mike's division rival in this league, I prefer to give the 30K foot view.
My team won the WS in Season 4 and the LCS in Season 5. I decided that since that team was aging, I would go into full rebuild mode starting in Season 7. So I plucked speed and defense from the Rule 5 draft over the next couple of seasons and played almost every position on the field with guys ratings greater than the the recs.
Long story short is that while I rebuilt, the speed and defense carried me, and all of the pitchers I acquired "mysteriously" overperformed from their career norms. Except for Etherton, who pitches great for Mike, but totally sucked for me (ain't randomness grand?). I increased my win total to 100 during the "rebuild" season and have continued to contend.
If you were to look at defense +/- totals and ratios over the past several seasons, you'll see without running a correlation that the top teams will generally
cluster around the top of either the total plus plays, ratios, or both.
I don't need a detailed to study to be able to intuitively see the relationship. If I had a guy like Snow on my team in Mantle (I can't get Jonas to trade him to me), I would probably DH him. If I was in the NL, I'd probably trade him, much as I would hate to, to an AL team. (BTW Jonas, you are welcome to take my advice and trade him to me - I'm in the AL
I think defense is very hard to quantify and the effects aren't really seen on an individual basis. This is only a hunch, but I don't think you see linear improvement by adding one or two excellent defenders. I think you see it in total context. One thing I regularly do is select my lineup based on my starting pitcher and because I value rest (kind of to the extreme), I shuffle guys around the field throughout the season. When a flyball pitcher pitches, I stack the outfield with better range, sometimes at the expense of the infield, but rarely since most of my players are excellent defensively. When a groundball pitcher pitches, the infield is solid.
I have pitchers that if you look at their ratings are good, but there are many better pitchers. I don't have a real bona fide ace, like most of the other contenders. I do have very solid to very good pitchers, but not to the same level as most of the Cy contenders, at least according to ratings. But every season, the staff, as a whole outperforms their counterparts who a lot of times have better ratings. Home park is a factor, but they're good every season.