Wire-framing isn't new. We've seen many before: Axure, Balsamiq, ...
To use wire-frames as a substitute for specifications, they must be 100% complete. A typical database project has hundreds of UI functions and wire-framing them all accurately and interactively using other tools would take weeks and cost a lot. So most teams can't afford this approach.
With VisualSpec you achieve that in hours, making it a perfectly viable approach to requirements specification.