The problem with Bess is that he's a liability on the outside. And when one of your starting outside WRs goes down with injury, you want your 3/4/5 WRs to be able to handle that spot.
In a world without injury, I agree that Bess should stay. But since injury is a NFL reality, we want our 3/4/5 WRs to be able to handle both outside and inside receiver positions when needed. Brandon Gibson, Rishard Matthews, and Ryan Swope are the type of 3/4/5 WRs that can be ok either outside or inside.