That’s quite an interesting question. For my part, I don’t think it may directly influence the judging, but in a way, these concepts may help push farther and quicker certain parts of the principal idea. I think also that this process is faster than to draw directly on the final layout.
You have a character that is in action on a set. You know aproximately the position of his body in space but you don’t have a very clear idea about his look. You have two options: design it directly in the main composition, or make some concepts sideline and then, use them as reference for the final image. Like you said, if it’s going to end up to be a 12 pixels background character, it may not be usefull to pass a lot of time on it and so, you should directly work on it. But if it’s an important part of the scene, I would definetely design it sideline. By doing this, you don’t have to focus on the perspective, the movement, the physics and the lights of the scene. All your thoughts and efforts are pulled on finding the shapes and the elements that will compose the character. The less distraction you have, the more efficiency you get. Once it’s done, it usually doesn’t take very long to reproduce the final concept on the final composition. And at the end, if all goes ok (that’s not always the case…O_o’) you’ve saved hours, you have a better concept and you can reuse the character in a different scene, now that you possess it entirely.
That’s what I think about it so far. Maybe I’m wrong, but it sounds logic for me at the moment.
Also, designing through concept is an activity very fun to do I think and that can help you improve your skills. It can also be used in a portefolio, like I plan to do with this work.