Thanks, I think I got my answer. My EDC light has nothing hanging off of it save for the easily broken pocket clip. (Are you listening Surefire?) My main "battle light" (Blackhawk! Gladius) has an "O" ring on it.
When I exit a store after sunset, the Gladius is in my hand, with the "O" ring on my support hand index finger. I'm a fan of "O" rings on lights.
I can see the logic now for using a light in a match with no lanyards etc, as that could easily be the only thing available in some instances. I don't see it as a shooting aid though. It's just a retention device. However, I did notice that the IDPA will allow the RO to fire up his or her light if a light is dropped during a stage to help the shooter find the dropped light. I don't know if that kind of assistance can be expected IRL though. I suspect though that's a nod to practicality to keep the match moving as well as safety.
I understand rules are rules. It's nice to know the reasoning behind them is all. I guess I'd better start practicing w/o the "O" ring thing.