What a photographer can do that a 3D illustrator can’t do
Though 3D posable people get more and more realistic, what 3D illustration can’t do well is people in motion. A person gets in front of a camera and they can be spontaneous—jump, run, smile, frown. Clothing flows with wind and gravity. Hair blows in the wind. In a few minutes a photographer can crank out hundreds of shots of a person performing in front of the camera and the art director has multiple choices of which shot works best.
In the movies, still today, if you want to make realistic animation of a 3D character, you put a body suit on a real person with various points on their body that are tracked by the camera, interpreted by a computer, and then the action is applied to the 3D character. Though software is always being produced that is improving the process, it still is time, labor, and cost intensive.
My recommendation, if photographic realism of a person is desired, use 3D characters when you have a specific pose you want for a layout, but can’t find a stock shot to match. A comp with a 3D model as a placeholder is a good substitute and more realistic than a sketch. You can explain to your client that if they buy the concept an actual human will be photographed. None-the-less, there may be times when a 3D character is appropriate, such as the male profile figure with the fungi models superimposed.
The point is, you have options. Sometimes, photography is the only way to go, but 3D illustration done well, is not only a viable alternative, it may be the best choice. (Click on image of woman to see more 3D virtual people.)