As Rebecca Dias mentions, my article ("Why WSE") covering the high-level reasons to use WS-Security has been published on MSDN.  It covers the provides benefits WSE provides, such as end-to-end message-level security, content-based routing, and policy through leveraging the WS-Security, WS-Addressing, and WS-Policy specifications. 

Let Rebecca know what you think of it.

If you want a longer article on the same material I'd recommend fellow-RD and web services enthusiast, William Tay's piece on Solving Real World Business Problems with Web Services Enhancements in .NET