If you look at the rate of plus plays generated per position per range rating, you'll find that each position has a different point of diminishing returns. In CF, for example, 90+ range is worth sacrificing offense for, but on the corners you don't get that many extra plus plays going from say 70 range to 90 which would justify a big dropoff in offense. (I'm sure dedel or booger will roll along soon and express this better than I did - but until then, hopefully you get the point)