Sure! You could install it on a domain controller/web server/application server/media server/any kind of server combination you want. The question is, why would you want to or why would you be constrained to?
However, we know what real life presents us with in certain cases. A previous company I was with purchased their first 2 Windows Servers for MS Great Plains. 1 was a mirror, so the other functioned as the SQL Server, Application Server, and Domain Controller. 5 Years later... It is still functioning just fine. GP has a max of 6 concurrent users, and the company only has about 8 or 9 users on the domain.