You do not gain Authority, nor SEO value for your main website if the Wordpress blog is hosted in a subdomain. I regularly setup "auto blogs" for my clients, wherein the Wordpress blog goes out via plugin and scrapes for keyword articles, spins them (rewords), and posts... This is fully automated, and is done in a subdirectory (ex: site.com/blog wherein the posts are used to gain organic traffic, authority, etc)

The current Wordpress options only allow sub-domain based installs which removes the entire point and does not rank your main website.