It could cure obesity: more muscles increases your resting metabolic rate. Just increase muscle mass to the point where your digestive tract is overwhelmed, and you start burning fat, no matter what you eat. Kind of like the movie "thinner", but this one would be called "muscular".
That brings up interesting questions about what happens when your musculo-skeletal system and your nervous system start fighting for calories. The stereotypical big dumb guy?
And isn't this gene therapy, where one shot changes your DNA for life?
Culturally, this will hit the fitness industry like a bombshell. Billions are spent annually on looking "ripped". Here's the real deal. Pretty soon, you won't get laid without it.