WordPress gives you access to using lots themes and thousands of plugins that will work under the XOOPS integration so your blog can look like it is the BBC or the New York Times, advertisements, or even the Dutch Marijuana Cafe Gazette
You are right, for a solely blogging site WP is the cms to choose. But integrating another cms as a module in xoops has a downside: will it support future WP updates? Is this module still compatible after a xoops core update? Will the module developer update this module till the end of times? Personally I would take a xoops module when adding a blog to an existing xoops website, it maybe less fancy but is more future proof I guess.