His site, his rules....
Facebook can set the rules, but they can't force people to play. When people don't like the rules, they do have the option to just stop playing his game.
I use facebook to keep up with my kids and grandkids, but while I've tried the facebook app several time I just keep uninstalling it because it sends notifications that I don't want or care about, even when I tell it not to notify me. I don't need my phone beeping and buzzing in the middle of a meeting or in the middle of the night because the facebook can scanned the friends of my friends, thinks that I might know them, and ask me to send them a friends invitation - or any of the other useless notifications that it generates.
If he reduces functionality by taking away messaging, then all he has really done is make his product less valuable.
Besides, anybody who I would actually WANT to talk to already has my phone number and/or email address, so why add a another method that doesn't honor the wishes of the user?