Many guests, especially the regulars, call the host to make a booking. This is good for the business. However, the host will end up answering tons of phone inquiries regardless of the time. Having the option to book online gives many advantages that are not only beneficial to you as the host, but to your customers as well.
After Airbnb went public listed, they are on a drive to increase their revenue. That is why the removal of guest service charges and moves to a 15% host commission model - up from 3% commission. Rising commission to the property is nothing new since most of the OTAs do so to the hotels over the years. Airbnb might follow the path to success set forth by other OTAs. The more hotel relies on OTAs, the harder for them to say "No" to the hike in commission. That's why many hotels focus on "direct booking" now to reduce their reliance on OTAs. On the other hand, the hike in commission is inevitable. After all, it's getting more costly for OTAs to distribute your Airbnb (or your room) because most of them are competing to sell to the same group of customers (fixed demand).