The idea is to keep Shred while getting rid of the positioning requirement.
Shred can be a proc based ability like Arcane Missiles. 50% chance on Mangle to proc Shred. This way you can't chain spam Mangle and still do top dps, but you wouldn't be forced to stand behind the target to do maximum dps.
Last I checked, Feral Druids are cats. Now if feral druids were dogs, I would understand Shred's doing it from behind issue.