I'd equate the manual trailer brakes to the emergency braking lever of the vehicle. I don't know many to have ever used it. But most would agree that it should be there within easy reach for that rare, hope it never happens, occasion.
That said, I do personally use mine, both the e-brake and the trailer brakes. Advanced technique if you will.
I've used the e-brake on my sports cars to drift, and the need to scrub speed situation when I police car is in view of my rear lights. Dumb reasons for sure, but I've used it on occasion. Not recommended as one could put their car in an uncontrolled spin if they don't know what they're doing.
Back on topic with the manual trailer brakes. I've certainly used them in tight mountain switchback roads where I absolutely want to be sure the trailer is not surging on the tow vehicle as I come into a turn, perhaps a bit hot. And at times in very high winds, when coming over a hill crest downhill into open cross winds, to make sure the hitch is in tension, and ensuring the rig is stable. I will say again it's an advanced technique. My setup is stable and I know my brake bias is correct between the tow vehicle and trailer. But it does offer a margin of safety if one knows what they're doing. Like the e-brake above, I don't recommend it for everyone as there's a certain understanding to using it. But better to have used it with practice, before that rare unexpected situation. My 2 cents.
