There are some common themes appearing in this thread. I've sourced all the parts for a RDB-16" 9mm, and am waiting on my smith's availability:
1) Access to indoor shooting ranges with limited rifle-caliber capabilities
2) Getting carbine-handling reps in during accessible pistol matches
3) Low cost-per-round training
4) Quiet shooting/Low blast
5) Less damage/closer range to steel or other targets
6) More accessible to low experience shooters
7) More compact carbines than 5.56mm guns
What else do we have?