Sean for me email is just one form of communication and it can be part of "getting the job done" but most of the time it's just noise. By limiting the use of email to 2 hours a day you are forced to prioritise what email you spend time on. By telling others what you expect to act upon you set a clear expectation what they have to do to get your time. It filters out all the noise like a secretary used to do. I find 2 hours a day way too much time if you consider that there's also meetings, video conferencing, telephone and other means of communication too fit into a working day.
This method also wouldn't work if you are a "foot-soldier" or "working-bee". This method only works if you have some form of power as manager where people expect you to set up rules and enforce them.
In the 3 years of being PM with this set of rules I did not miss an important email once by the way.