MS needs to make a version of Windows Server that supports ARM, with roles restricted to secondary AD DCs, DNS, DHCP, and routing. That would make them perfect for telephone/LAN closet authentication and network services with very low power (12-15W) consumption, right along side the local switches. Perhaps they could even get built into the switches. Admins would still administer from a central DC, but it would take the authentication load from that central server.
Many companies have moved this kind of thing to virtual machines, but that still requires big, power (>250W) hungry hosts. With smaller, low power units based on ARM, they could reduce the datacenter footprint, power consumption, and heat, spreading it out to telephone/LAN closets.
The main DC would still have to be a big x64 machine, and so would file servers, print servers, DB servers, and other services, but ARM could have a significant contribution to the datacenter.