Stephen – you mentioned your focus is around security? How are you controlling access to your Exchange environment (requiring certificates)? If no certs or MDM -how are you limiting your EAS traffic to just the folks with Samsung Jacks. Exchange provides some tooling to see who is using ActiveSync but it leaves a lot to be desired. From a security perspective you should want to know OS, make, model, inventory (hardware/software), firmware version, etc including how many devices each user has registered and when the last date each device has connected.
Without this control & visibility you can also run into Microsoft license compliancy issues with over-deployment.
This chart below shows the features available by OS with EAS management and you can see once you audit your system and you see Android and iOS connecting you’ll definitely want MDM.
http://en.wikipedia.org/wiki/Comparison_of_Exchange_ActiveSync_Clients
Many MDM vendors will be able to do additional Encryption controls over and above the native EAS. Things like Data Fading if the device hasn’t checked in for XX, Selective wipe of PIM area, external media, specified folders, encryption types.
There are also config features like application whitelist, black list, control panel access, etc.
There is actually a ton more that a MDM provides but it really depends on whether you feel what you have in place is not enough. Most people start with Exchange ActiveSync but then determine they need much more.