Its the other way around "WCM authoring" deploying to "alfresco".
There's a few reasons, some of them better than others :?
Here's some of my reasons
a) because the deployment refactorings of 3.0 and Community 3.2 have made it far easier to implement.
b) because the content is then available via CMIS rather than bespoke or alfresco specific interfaces.
c) because it gives access to the other alfresco services like transformation.