Value that Prism offers is immense... far too many developers have constructed "big ball of mud" without any code structure. Prism provides simple way to apply proven patterns and practices for building extensible, module and maintainable .NET application that can evolve with requirements, and withstand test of time. Having architectural guidance such as Prism improves image of solutions build in .NET. Application without necessary code structure look like a product written by struggling amateur.
Practicing concepts contained in Prism library, help me to build better server side code (modularity, component discovery), and be more effective software developer. I consider Prism essential part of any professional .NET application developer toolbox!
Very glad to hear about potential of Regions available on WinRT and Forms.