Where I live, there are still a lot of AM stations. Even FM stations broadcast mono, with a stereo overlay. I don't know the exact specs and such, but when your FM signal goes weak, it drops to mono pretty quickly. Why would anyone want their song to fall apart under those conditions. With the little effort it takes to get it right, why not? It's not going to hurt your stereo mix.

I will always check mono compatibility! I want the music to sound the best it can even under the worse conditions! I have yet to hear a good arguement to the contrary!

Just my 2 cents!