The advantage of playing slide on 12-string is that the string tension is such that you don't have to raise the nut to play cleanly. More difficult to do on a six-string, where, with the slightest pressure, you'll inevetibly hit the frets.
And -- unless you're playing on a guitar with a flat fretboard (like some of the early Guild 12s), this works better if you use a slide that is slighty concave to match the radius of the neck.