I think many of us on this site have jobs where presence and productivity are pretty elastic.

Many workers have jobs where human presence is a key part of the job function. Being a cashier, or a nurse, or driving -- it isn't as though these things happen unless you are physically there. The quantum of human presence is "one human" :-) so even if there are no customers, the driving is boring, or the patients are asleep, it still requires you to be there on the job.