Over the years I've seen a lot of people take bad face shots that range from busted/bruised lips to missing teeth and broken jaws. I've even heard someone wearing a FF who thinks he would have been knocked out upside down had he not had one on. Given that I'm an advocate of FF helmets for many WW applications as I think the benefit far outweighs the downsides.
It is somewhat more difficult to communicate with a FF on. However, almost every exploratory run I've ever done was wearing a FF and I would call it a minor inconvenience. Generally speaking you're either using hand signals, yelling very brief instructions, or in a position where you can lift your helmet to communicate more complex instructions. If you're a river guide I could this might be a big issue though.
For beginners I could see an argument for not wearing a FF based on being in slower, less dangerous WW. That said, my very first day outside in a kayak, I went to a playpark and window shaded upstream. My face smashed directly into a rock (I survived though).