Yeah...it's true that things are not alway played in stereo, but at some point it's gotta to be an either or thing, and you go with it.
I'll be honest, mono mixes are not my consideration at all, just like surround mixes are of no concern to me. I'll check mono for the sake of checking, but a mix to me isn't just a question of making it work in all other possibly playback systems.
Not to sound snooty about it...but I like to think of the mix as the final creative production decision, and for me it's always a stereo mix...so panning is a big part of that and it's not just to fix mix problems. It's about laying out the virtual "stage" for the listener. If the listener chooses to change the stage to a mono listening experience...it's on their dime.
That said...I find that most times, if the stereo mix is balanced well and panned, and levels and EQ working as they should...when you collapse it to mono, it's still pretty listenable...but certainly not the same as if you had specifically mixed it for mono.