What I struggle with as far as pricing my Airbnb is it can fit 6 people so I see it and the market dictates that the fare price is roughly $600 a night. But if I list it as $600 a night now my place is to expensive for the couple, just two people, looking for a place. So I see it as. Advertise for $200 a night for two guest and then charge an addition $100 per guest and in this way I have an option for larger parties and option for couples. I think this may answer your question of why charging per person makes sense .
The only gamble on my part as a host is if the couple (2 people) rents my place months in advance then that's leaves my place not available for larger parties equalling missed income.
I believe the solution here is to advertise as $600 for the night and then slash the price down for last minute bookings. Interested to hear others thoughts.