1) Shorter URL in general. 2) Easier to change technologies or move the blog to different providers. 3) Flexibility in choosing which technology to use for your blog: If your main site is built on Ruby and you want to use Wordpress for your blog, subdomain is clearly easier. 4) Still has some subfolder SEO benefits. You can still pass some value back to the root domain through cross-linking. 5) Achieve higher penetration on search results, since subdomains are treated separately from root domains.