Playing slide guitar is to playing guitar what ice skating is to walking in comfortable shoes.
Fretted instruments are more likely to be in in tune. Leaving the frets becomes a delicate balancing act, slipping between self expression and skreech. The bar you hold to make the notes can turn your fingers into hulk hands. So why bother playing slide? Because just as the skater's form is suddenly free from gravity, the slide player's notes glide up and out of pitches with a sustain that some of us find electrifying.