I hate the thought of the opportunity cost of developing a fully integrated CMS. It's a lovely idea, and I don't necessarily object to the so-called "bloat" that some people complain about (which is rather misleading, it basically assumes that the XF developers are bad at their jobs). The biggest problem is that a full CMS is going to cost a lot of man hours that would otherwise be spent on the core forum platform.
I think a good integration with WordPress would be a far better solution. As mentioned above, even if the XF team worked like crazy and could release something that had feature-parity with WordPress, they still wouldn't have the giant collection of themes, plugins and thousands of blogs/webpages/etc with support or tips on the system. I mean, one of my favorite things about WordPress is that I can plug "wordpress <problem description>" into Google and have an immediate solution in the first 5 results.
A good API could facilitate the building of a bridge between any number of content management systems. And hey, with a high quality API, you can, as they say, eat your own dog food and use that same API to build a full CMS add-on if that becomes desirable.