That "if it passes the pull test" jive only works if somebody really applies the full pull test every time. We know better than that.
The way the Stunt people do it, they are. At least, at the Nats (where they have that bucket-and-pulley setup) they do.
One might make an observation, though, that pull tests don't help when an airplane comes in and then jerks on the lines. Pretty much all line-tension related breaks I've seen (in Combat) are due to that (and other flyaways are from line-tangles and cut-aways).
One reason to use heavier lines voluntarily - they don't stretch as much.