Hi there,
I had just the opposite reason for needing a phone or email, but for the comfort of the guest. They wanted to contact me directly to ask a few questions prior to booking. It was very frustrating, to say the least. I couldn't share my phone number and they couldn't share theirs. I'm not sure why the guest didn't want to communicate thru Airbnb, I think it just seemed easier to speak in person to ask the questions (old school - haha). I tried everything, including spelling out my digits. We even tried thru VRBO, where I am also listed.
My guess is that it is to eliminate the possibility of booking buests thru airbnb advertising, but skirting their system and payment. Its too bad though, as the guest got so frustrated that they ended up no booking at all. I was dissappointed as it was a prime week for me.