My recollection is that Freedom of the Hills lists rapping through nylon slings as standard procedure. True, the melting point of Spectra is lower than tubular webbing, but if the rope remains stationary (i.e, you rap slowly and carefully), the risk is probably (I said PROBABLY) not too great. I have heard of people top-roping off this set-up (doing so ignorantly after someone else had rappelled), and cutting through. Of course, I'd never use such a sling again after pulling the rope through it! But really, we're generally talking about a situation that would only likely occur when bailing off a long alpine route, right?
we're all living proof that nothing lasts