Speaking for myself, never use a swivel on a spoon fly, such as Dupree’s or Horbley’s.
First, in my experience and when properly used the spoon fly if fished or retrieved excruciatingly slow so as to wobble back and forth. It only twists if fished too fast ( or through a strong current not the best place to use such a tactic imho)
Second, though the take on a actively moving spoon fly is usually obvious, more depth and a slight line sag already put you further from feeling the take. The addition of the swivel puts another potential isolator in play between you and the take. For me, feel is everything no matter what tackle I’m using. Whenever possible I try to keep the joints in my rigging to absolute minimum. I learned to tie great knots and to ties them fast. Loop to loop knots make for very fast changes too as long as fish aren’t leader shy.
Third, the extra weight of all but the smallest swivels adds weight causing the light weight spoon flys to drop too fast, which may be welcome sometimes, but for me, slowly working shallow early am water I want less weight.
Forget all that above, if you’re dawn patrol working a large spoon into the surf at daybreak. The Coastal snap swivel is my best friend helping to minimize the inevitable line twist during rapid fire cast and retrieve. When blind casting arties, I’m covering a lot of water quickly and methodically, but after 10 mins with no hits ( and I just know there are keeper fish present ) I tend to shift my lure in a kind of fish finder fashion. I try to stay off the bottom too
Swivels just make that method easier than knots most of the time. With Mirror lures, and Corkies, etc., the trebles and swivels occasionally get in each other’s way, especially into the wind, but still worth it to have them on there imo.
That’s me anyway.