I think the takeaway is that it lets developers be the designers. Most programmers I know are pretty good at solving problems, but they don't always have an eye for visual details aspect of it. The style guide gives them a framework to work in, and can hopefully prevent a UI atrocity.

Maybe hiring an outside designer to do the whole app would get you better results, but if you're going to toss all of that as the app changes, it might not be worth the expense. Assuming your devs aren't bad at user interaction stuff, this gets you most of the way there at a small fraction of the cost.