For guitarists only?! HA!
While the pick slide is most often done using a pick or plictrum on an electric guitar, this is by no means the only option for the effect. Pick slides can also be performed on an electric bass, in largely the same fashion (scratching the pick rapidly up a string, most of the way up the neck, controlling the pitch of the sound using the speed of the slide). Bassists who use pick slides include Lemmy Kilminster of Motorhead, Sepultura bassist Paulo Jr. and, in fact, many heavy metal bassists. The technique yields a different sound on a bass than on a guitar, and is indeed much deeper and darker in sound on a bass. The technique can only be used convincingly on a bass strung with roundwound strings, flatwound and pressurewound strings won't work because of their smoothness. Using the technique on Nanoweb-covered strings (like those made by Elixir) will make the fragile Nanoweb covering fray, causing the string to lose tone and become very uncomfortable to play.
Pick sliding without the pick
Bass players who don't use picks can make a similar effect with a little more difficulty and considerably less comfort by using a fingernail in place of the pick.
Pick sliding then and now
On a guitar piped through hefty distortion, the pick slide yields a "sci-fi flick laser beam" sort of sound. This is how the Slayer guitarists Kerry King and Jeff Hanneman make all those wierd "laser blasts" in the intro to "Killing Fields", and how Ozzy's guitarist Randy Rhoads made the same sort of effects back in the 80's. Pick slides are sometimes used in metal and rock, although the effect was so overused during the 80's that most musician find the effect itself to be hopelessly 80's today.