One of the reasons, according to Scott Ambler,
Why Agile Software Development Techniques Work, is that they reduce the feedback cycle between the generation of an idea (perhaps a requirement or a design strategy) and the realization of that idea.
Of course you can argue on that, and that's exactly what Scott invites us to do!
To give one away, the reduction of the feedback cycle is not very worthwhile if you just want to fully replace an existing, outdated software system.
(by the way, I think RUP, carefully applied, is Agile).