Why does it matter that a middle-class tipping point has been reached and that the middle class is the most rapidly growing segment of the global income distribution? Because the middle class drive demand in the global economy and because the middle class are far more demanding of their governments.
FWIW, I agree that reaching 50% globally is of little importance, but it's certainly very meaningful locally. China's meteoric rise in the last two decades was born from the rise of the middle class, and the same is now starting to happen in India and Indonesia.
Also, And while the rise is not without it's downsides (pollution, obesity, whatnot), as the authors note that it's quite well established that escaping poverty/vulnerability makes people a lot happier. Risking a heart attack from a bad diet 30 years down the road is preferable to starving now due to a lack of food.