i believe there was a previous explanation for this, i quote jrwhitehead.
The Mobility Master is a new component of the Aruba architecture that enables customers to take advantage of advanced features that require central coordination and for networks to scale due to increased demand for mobile and IoT devices. It also replaces the prior functions of the master controller and can be either deployed as a VM or an x86 hardware appliance. The Mobility Master provides automatic RF optimization and enables hitless failover in an unlikely event of a controller outage.
The main thing is you have to decide or know if, in 8.x you want to leverage the features on the Mobility Master (Clustering, NBAPI, Airmatch, etc) or not. If they are not critical to your need, then you can go with 8.x on a controller without MM. If you do want those features, you need the Mobility Manager.
Link
http://community.arubanetworks.com/t5/Wireless-Access/Mobility-master-and-VMC/td-p/285829