Some folks think that for very picky fish in clear slow currents floating tippet actually spooks fish. The floating tippet depresses the film ever so slightly, and the tippet itself acts as a lens that refracts light. This causes both reflection and refraction of light which some feel spooks the largest and spookiest fish.

Since flurocarbon sinks and is has just about the same refractive index of water, it actually is less visible than floating tipper. Also in these slow flows, water just below surface drifts at the same speed and direction as water at the surface. There should be little difference in drag caused by a floating or a slightly sunken tippet.

I've never fished where this type of condition is common but I think it does occur. Food for thought.