Just a few thougths.....with 4 crew (even 5 on occasion) on 73's, if all crew are out serving the pax, it can be quite hard to contact the cabin crew in critical (but not emergency) phases of flight. Sometimes we need timely communication with the cabin crew, without needing to go for all the bells and whistles.
If there is a passenger disruption, it can take all cabin crew to deal with the situation, in the meantime the flight deck door can be exposed to additional security issues.
So whilst sebby can do a fantastic service on a short sector, serving sambos with only 3 crew, I think there are further considerations which need to be accounted for.
After all pax service, whilst important, is really only a secondary priority to pax/crew safety.