I would love to be able to use the custom arp to trigger external gear as assignable controller messages. In other words, let's say I am using the virus TI as a control surface for another synth in ableton, and I link the virus to control a parameter that takes midi controller 57 for example. Now I would use the arp info to trigger values from 1 - 127 for controller 57.
Example 2 would be to use it to control other gear, for instance my Roland TR-8 accepts controller information for things like drum Pitch, so I would set my controller number for the arp in the matrix and have it generate the controller values on midi out to my drum machine. Voila, a talking drum 
Just a thought, but a great way to add some life to my other gear (which is nowhere near as awesome as my TI2)
MrMowgli