Hi Dingo
I think it is down to the amount of people who will be accessing these functions.
If you are an organisation of hundreds of people, all regulrarly accessing files and emailling, a single server may not be able to adequately supply enough power for both functions.
Another advantage of separate servers is that they can be restarted independantly.
I look after a number of servers in my organisation of sixty or so people. Our main server is a twin 2.8GHz Xeon with 2GB of memory. It is more than adequate to serve the demands of the Exchange server software and file & print sharing for this number of people.