I only answer my Facebook messages BECAUSE I can answer them on my mobile app. I won't install another app just for Facebook messaging, so I guess those messages will never get answered.
The inclusion of chatheads systemwide on Android makes it a great experience for me (of course YMMV). I wish FB Messenger and its chatheads could handle my SMS as well.
Why not?
I imagine they are doing this because they don't see the point in supporting both code bases.