14 teams x 6D = 84D in play. Last year's (waiver wire) D scored 21pts.
So a 45pt (Shattenkirk?) defenseman does represent a +24 over a WWD.
14 teams x 12F = 168F in play. Last year's (waiver wire) F scored 36pts.
So a 65pt (Eriksson?) forward does represent a +29 over a WWF.
This is why I think you have to consider keeping an Eriksson over your D.
Eriksson (70pts) represents a +34pts over WWF.
And, as you say, if good D are "valued" highly... you should be able to sell them for a great (draft pick) return.