I am a long-time user of the non-slip mono loop having been taught it by a guide friend in the Keys back in the 80's. It is my go to knot for all saltwater flies imitating bait fish or crustaceans. It is also applicable for freshwater streamers and, though I do not fish nymphs much, why not particularly for something big like a Pteronarcys stonefly.
It is a great knot because when a stripped fly is paused the fly waifts less encumbered then it might with a fixed knot on a heavier tippet. Larger bonefish and all permit take a crab or shrimp fly on the drop as the imitation suggests a natural organism heading toward the bottom to secure itself in the marl.
Imitating the natural attitude of a mayfly or caddis imitation is the opposite situation. Firstly, you are using a fine tippet and not stripping. Secondly the fly is small and a far less bulky knot generating minimal surface refraction is required. Most importantly it is YOUR job to position and induce a natural dead drift of the fly sans vagary of the fly swiveling in the current on an uncontrollable loop knot. Indeed, the opposite is required. A knot that is not only fixed to the eye but will resist rotating on the hook. A through the eye twice Trileen knot is not only very strong but even more relevantly, locks in place, resisting even under the pressure of fighting a fish, moving to the side of the eye. This reduces frictional energy which damages Nylon as well as preserving a fly first attitude on the currents. The tippet to fly knot does not induce a more natural dead drift, the anglers' articulation of line and leader in air and on water does.
