That depends on your goal and the quality of your staff. If your staff fulfills your goal then no, of course you shouldn't hire a perspectivist. However, I find many firms that have a person or two that create 3D models just don't have the background, time, or resources required to create quality presentations. If your project requires an illustration for marketing, sales, or client/city approval, then quality is very important. You wouldn't hire a handy-man to build your next project would you?