It's not how much time a professor spends in her office, it's how she spends the time that she is there. This presupposes, of course, that a professor must be in her office for a certain amount of time. And, I agree that she should. It is not wise to require research professors to be in the office four days a week.
Research requires thinking and thinking requires leisure.
And, having to do this or do that on a given day when one needs to think and write interrupts that process. If professors are required to come to campus and to sit in an office, that could stifle needed time away for thinking. For example, as a graduate student writing the dissertation, there are days that I literally do not leave my apartment. I do not respond to email. I do not answer the phone.