I use the fluoroclear because they don't (can't?) make a fluorescent line in pure fluorocarbon - I gotta see the line or it drives me insane in the membrane.
Why not put fluorescent dye right into the PTFE like they do mono? I am guessing the fluorescent dye molecules don't entirely dig the PTFE matrix. When added to PTFE line they probably don't go into solid solution. Rather they transfer their excited state energy to the PTFE matrix immediately after excitation in order to get back to their beloved ground state - rather then emitting fluorescence.
Fluorescent dye molecules do fluoresce with tremendous quantum efficiency when in the nylon solid matrix (traditional mono) however - so the PTFE coated nylon (p-line's fluoroclear) delivers the hybrid performance I need. Fluorocarbon behavior with mono fluorescence.