The best sound is not from neither the mouth nor the chest, it's from the golden spot in between. See, the mouth gets you all the treble you need, and then some, whereas the chest is all core and bass. By forgetting your eyes altogether, and giving all your focus on the sound using the eyes for nothing but coarse guidance, you can find a softer, fuller, more natural, and in less need of a polishing sound. Too much mouth, and the sound will get thorny, thin and sibilant - too much chest, and the sound will get muddy and distant. The more the editor will have to do afterwards, the more of a failure the recordings has been.
When using song-microphones we don't have this problem, these microphones are mainly modeled to avoid excessive sibilance, especially large diaphragm ones, and the proximity effect takes care of the thinness. In movies, we never ever get that close, and if we do, we better not use it.
The downside though is that you must keep that very spot no matter what - The same distance, the same angle, the very same position to the sweetspot. Even small deviations lead to great changes in sound characteristics, meaning you have to know what your doing of it will end up unusable. If you can't get a clear line of sight, and can't move freely, then you're in serious trouble. Same goes for if you use a too long a boom and can't direct it with enough precision. This is an artform in itself.
It's not an easy thing to pull off, you have to mind both the camera, lighting, acoustics, and a lot more variables than I care to line up right now, but there's a reason this is a real job and it's the dumbest idea imaginable to not be using a dedicated and trained boom-operator for booming sound.