If we were born with this stuff in our bodies I'm sure it serves a beneficial purpose. It's just not something that we have that limits our ability to grow muscle - there has to be other effects.
Either way, the article was very interesting.
Given that animals and humans who don't produce (or are insensitive to) myostatin appear to be otherwise healthy, I think it's a safe bet that myostatin's main function in mammals is to limit muscle growth. Muscle is metabolically expensive to build and maintain, so inhibiting unnecessary muscle growth would be advantageous when high-calorie/high-protein food is scarce.