Philosophy tomes spent on my Gryphonskin set: 10,125
Philosophy tomes saved by NOT buying Darklight for ANY physical DPS classes (I did buy the tank DL first): 5,625
Armory slots saved by not having 4 sets of jewellery instead of 1: 18

Damage increase by using Gryphonskin instead of fending: 15-20%
Extra healing on Inner Beast: 15-20%
Extra parry chance gained? Depends on the effects of DEX on parry. Unknown but best guess is a % or two.
Extra damage reduced when parry succeeds? 1-2% (minimum 1%, but can hit 2 caps depending on where you are)
+30 crit or DET is easy to get over the set as well.

What do you lose? 375hp(ish) and if DEX doesn't affect parry at all (we don't know) then you lose a small amount of parry.

Anyone using Allagan <blank> of Fending in their jewellery slots is either doing it wrong, fighting Twintania or just hasn't made their Gryphonskin yet.