Redefining Prophesy

As I've said before, I consider many 'practicing' charismatics to be brothers and sisters-in-Christ. Even so, I'm still unable to reconcile the habit of redefining prophesy as illustrated in this post, "Spontaneous and Prophetic Songs" (Please remember that questioning, examining, or challenging a teaching and/or a practice in light of Scripture is NOT an attack upon the person.)

I've always wondered--even during my days as a 'full blown" charismatic--what this practice is intended/hoped to accomplish that Scripture apparently is assumed to be deficient in?