JerryM, Mas Ayoob had an article about a jewelry store (I think in NC or VA) where the shop owners had prepositioned revolvers behind the counter, and ended up using several of them in a shootout with two armed robbers who had entered the store firing their weapons. So there's one, anyway.
My personal take on BUGs is, if you can reasonably carry one, there's no reason not to. Reasons for having one have already been listed. I may have missed this one, but it is another consideration of mine: It's nice to have one for those conditions where I know the primary will be harder to access.
For instance, it's winter in Missouri. On cold days, I wear a coat. A coat makes it a bit tougher to get to my IWB auto; the 442 in my outer pocket is much more readily accessible.
(Note: I usually keep it in the left coat pocket, my weak side, and I usually practice a lot of left hand shooting with the 442. That way, in the event my right arm is disabled or grabbed, my free hand has a gun available, and has a lot of muscle memory with that particular gun.)
Or my best friend, who is a lefty and usually carries a 9mm in left-hand appendix IWB. He spends a fair amount of time on the road, and realized that drawing from that position, in the event of a carjacker at the driver's door, would be virtually impossible. He carries an SP101 in a right ankle holster when driving.
Edit: He trains with his right (weak) hand with the SP101, primarily for the ability to shoot cross-body in the vehicle, instead of trying to wedge his left arm between his body and the door.