What is a shift slide?
In a shift slide, a note is fretted, then struck, and then the fretting finger slides up or down to a different fret, and the string is struck again. A legato slide differs in that the string is struck only for the first note. The first slide pictured is a shift slide; the second is a legato slide.
Is slide bad for guitar?
If your slide is hitting anything on your guitar besides the strings you are not slideing correctly. You can dent your fretboard if you're careless. That's one good reason to avoid playing slide when you're drunk or otherwise wasted.