[Sharlin]

Because taking gamma correction into account when doing blend operations is pretty vital if you want good results.

[raphlinus]

Also because it's by far one of the most common things people get wrong, or could do better, when implementing alpha blending.

[mark-r]

You can get 90% of the way to correct just by using square roots and squares. There's really no excuse.

[iamdead]

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).

[jacobolus]

People currently shy away from making images where the current hacky method gives bad results (because nobody likes making ugly images). If implementations were fixed, they might not.