Their core business is making two private parties feel good enough about each other in order to conducts a higher value transaction.
Friendliness, trust, professionalism are all part of the image AirBnB wants to convey to guests and hosts. I would say that a font is extremely a part of their core business as anything that even slightly improves the feeling of confidence you want your customers to have when they book a room — improves conversations, perceived satisfaction and a whole host of other intangible, but still very real aspects of the AirBnB experience.
Spend some time as an AirBnB host and you’ll quickly understand their core business is beyond just being a transaction engine but more around almost being a therapeutic interface between hosts and needy, insecure and hesitant guests. Remember also that AirBnB is growing rapidly — a large number of hosts and guest have never “done this sort of thing” before. So AirBnB’s design motifs have to be psychologically perfect to make those people, like your Aunt Edna, feel safe.
For the average HN reader, certainly Menlo, Times New Roman or Comic Sans would be suitable.
Design matters and it matters a hell of a lot when you are dealing with the “normal” public.