Watch the Vocal-like Phrasing & Expression online guitar lesson by Lee Roy Parnell from Slide Guitar Slinger
One of the things that drew me to slide guitar first was the expressiveness of playing, and the ability to not be directly on one note or another. B.B. King said he would have loved to play slide guitar, but he couldn't get the vibrato down (instead developing incredible left hand vibrato that looked like a hummingbird).
Being a singer first out of necessity, the slide and voice were synonymous to me. They express the same emotion in the song. The slide on guitar is an extension of my singing voice, meant to augment, not replace. There are several instruments that have this quality, but none of them are fretted like the guitar!