It's more a stylistic thing / linguistic tic than about the savings in characters. Without would be "w/o". Like, "I'm going to the dance, w/ or w/o you."

Or imagine an album titled "W/ or w/o you". Has a nice look to it. Also, check out the Google results for "w/ or w/o you". Impressive expansion on the part of Google.

(Edit: I originally said it saved three letters, but of course, that was me forgetting to count / as a letter. :)