Having AI units fighting alongside you, even if they were limited to certain areas or as AI units in a different Game-type (ODST Squad in Halo Reach - Installation 04 map, anybody?) would be good, say... Police/Army 'creeps' compared to the more mobile/intelligent player characters? For lack of a better term.
Theoretically, they could spawn at the start of either the game, or per-wave at a limited number, and either move to a predetermined position/designated area in-editor, or follow after a PC, fanning out and keeping back from the action, using lower-level equipment to encourage PC's to be active. There could be potential for sending orders to AI units to - and this is merely speculated/my humble opinion - areas on the map marked as Alpha, Bravo, etc, follow, stay here...
That's all, if of course, AI units are implemented somehow.