A 3D rendering application is a tool - and one of many available to the digital artist. To say that you should be restricted to that is absurd. It's like saying that an acrylic painter can only use a bsable brush, but not an airbrush. As everyone above has said, the ONLY thing that matters is the end result.

This certainly isn't the first time I've heard of this "limit" to how the digital artist should work. The only thing I can figure is that it's be promoted by those who don't have enough conventional art skills to do proper postwork. Whatever the case it's a silly notion.