The argument that pures can switch specs depending on the encounter would feel more justified if it weren't such a hassle and investment to swap gear (I'm thinking of mages in particular). I do feel that hybrid classes should have everything in their toolkit to be able to handle every situation since it is their only dps spec available to them. however, while having the tools does not mean that they will be the best at every kind of encounter. Shadow is missing tools and feels incomplete with the shadow orbs.
Shadow needs something to identify itself away from Affliction as they had everything shadow had to offer (dps-wise) and better all expansion.