This topic came up on another forum... and quickly turned into a bloodbath of an overly-opinionated, under-informed shouting match.
I've heard at least once or twice that rapping with your rope through a Spectra sling is dangerous. I always assumed that this was because of:
1. The low melting point of Spectra, and that a short shift of the rope in either direction could cause enough friction (under load) to start degrading the sling.
2. The limited surface area of the sling presents a greater risk of tangling the rope than is the case with standard, nylon webbing.
3. The weave of the Spectra material could actually create a situation in which the fibers would cut the rope.
Give me liberty, or give me death!