Our customers typically use the Elk for security and will generally run the most fundamental bits through the Elk (HVAC and Lighting), and of course it can provide various types of I/O for sensors and such. Then they layer CQC over the top of the Elk. CQC can get to those other bits via the Elk, and then provide the sexy touch screens, media management, control over all the stuff the Elk doesn't handle (home theater, multi-zone audio, media devices, etc...) and things like weather data, traffic cams, much fancier text to speech, and so forth. And they often use CQC for higher level logic, which is a lot easier in some cases to set up in our system than in the Elk.
So an M1/Omni plus something like CQC makes for a nice combination. CQC could directly interface to all those things, and some people do it that way as well, adding in a security panel to replace the seucirty aspects of the M1/Omni. But, if you are going to end up needing a fair bit of digital/analog I/O, you have to provide that somehow and these types of boxes do that, and they provide security. So a fair number of people go that route.