Casting those light jigs is where it's useful, but the guys fished with at Taney that used 2 lb. leader used braided mainline, but I think I remember them saying it was 3 lb. braid.
Fished with Milo in the trophy section, he was mainly throwing jerk baits, but picked up his jig outfit with that light line and caught 5 trout on 5 casts, I was throwing 1/8 oz jigs and catching one just every once in a while. Maybe it was coincidence, or a fluke, but in that short time frame the light line and 1/16th oz jig was king.
Later that day we fished below the catch and release area where we found concentrations of fresh off the truck rainbows. They did not care about weight of jig or size of line.
I'm in the camp of it's not the visibility but the castability and presentation of lighter line that makes a difference.
4 lb. test is my threshold, I go no lighter than that. If people are whacking them with 2 lb. test and I'm not, then I will be happy to be their net boy.