I spent sometime in the film vfx world before moving into ID. This trend may be new to the ID world, but it's a common technique used by 3D film compositors. It's used to match 3D to film plates during post production. The reasoning being 3D renders often come out looking to perfect and clear compared to the film footage which often has sometime of grain associated with it. Another trick was to use an edge mask to slightly blur the edge of 3D assets to break up super sharp pixel transitions with film footage.
Every photo-realistic render should have some type of grain, as many other's have said in this thread we are simulating a camera photo. That being said it should be used with a reason. Close up or detail shots on small and even large products should include noise along with DOF. In context product shots should also include a small amount of noise to match the photo background. Pretty much any other type of product shot can include less than .75% noise to just break up that perfect render look. This all being said it's a stylistic choice for many designers but these are the rules of thumb I was taught as a VFX artist.