The Mobility Master (MM) is a management platform that manages all of your MMs and APs using a hierarchical inheritance structure. APs do not and can not communicate with the MM, they communicate with the Mobility Controllers (MCs). The MM also performs other centralized tasks for the network and allows other capabilities, such as; AirMatch, Clustering, Live Upgrade, and MultiZone, to name a few. Deploying ArubaOS 8 in an MM environment is the best way with the most features.
You can also install ArubaOS 8 as a standalone. This standalone can operate with a VRRP backup, it you need redundancy. In this configuration, the APs will connect to the standalone controller. This configuration will operate similar to the way ArubaOS 6 operated. Most of the new features of ArubaOS 8 will not be implemented.
Lastly you can install ArubaOS 8 as what is know as a Mobility Controller Master (MCM). In this configuration, one controller operates similar to how a Master controller operated in OS 6, and one or more controllers will operate similar to Local controllers in OS 6. A difference is that in OS 6, the MCM CANNOT terminate any APs. So, the one device is dedicated only to management. The additional controllers will terminal any APs. In OS 6 it was not uncommon for APs to terminal to Local controllers, and failover to the Master controller. This is not possible in an MCM environment.
So, yes, you can install ArubaOS 8 without a Mobility Master, however you are much better off deploying it with one, because of the added new features (Clustering is hands down a reason to do it).
I hope this helps,