To the contrary, non-gamma-correct compositing is very common, since so many programs (both historically and today) simply never bothered to do it correctly--but we get good results anyway. In most situations we wouldn't notice the difference, it usually takes a somewhat contrived example to illustrate it well. You can see that the example chosen blends between primary red and primary green, which is not a natural color combination. If you replaced one of the two colors with primary blue, even then the artifacts would be less apparent (because of blue's lower luminosity).