As far as I'm concerned, and as far as opinion matters...mixing through headphones has a proper place and a proper time.
Beyer makes headphones that you can aparantly mix with all day and suffer no listening fatigue. In addition, I've been told the stereo field in these headphones are reliable to a degree.
To me that sounds better for people who need to do field recordings which require an onsite application where speakers can't go.
People also forget that our ears don't wear out evenly, which affects our natural abilitly to triangulate sounds. And that becomes amplified when using headphones.
I personally have a little more hearing damage in my right ear. So if I'm reaching for headphones, I'm strictly referencing things in mono to the mix up on the speakers.