We have found that it is very difficult to evenly light a backdrop using floor stand lights. The edges tend to be too bright and the middle of the screen is too difficult to reach because the live action tends to catch much of the light intended for the backdrop. We find that floor stand lights take up too much of the space needed for everything else that is needed for a shoot. They compete for space with actors and the props as well as cameras and other lights needed for the live action. It quickly gets to be cumbersome to have to keep rearranging assets so that they don't get in the way of the camera and the director's view of what is happening on the stage. Hanging the Chroma Key Lights from the ceiling releaves a lot of this conjestion. They do a better job at getting behind the action and putting uniform elumination where it needs to be. Our green screen lighting systems make a lot more of the stage available for everything else that compete for space. Actors can get very close to the backdrop without casting shadows onto it. I really appreciate not having to mess with floor stand backdrops lights any more.
The mirrors in our backdrop lighting systems serve to shade the actors from the backdrop lights while doubling the lighting onto the backdrop. Needing fewer lights to light the backdrop means there will be a smaller heat load in your studio keeping it cooler.
If you are fortunate enough to have a two-story chroma key studio then you can light the backdrop from both the ceiling and the floor. Floor lights can be hidden behind a blind that is there to obscure them from the camera's point of view.