WHEN TO USE A DOMAIN CONTROLLER

Hi,
My client has 17 offices, most of then has less than 10 users, and they also have HQ with 50 users.
Their current configuration is to have a site per office with a domain controller on it.
In each office the domain controller is also a file server.
My question is, considering the file role is more important than DC role, is it necesary to have a DC per office when the office are small?
What is the network traffic of authenticating users vs AD repliation traffic? My users only use, outlook, and a MS SQL server based applciation.

Thanks