In my experience, adding a blog to your site can be beneficial as it allows you to constantly update the content using phrases and terms your target market searches online. In this way your blog can help your website increase in search engine rankings because it is another portion of your site that constantly receives new content (which is imperative to helping SEO efforts). Also, if you are interested in getting marketing your blog, I am a member of www.triberr.com
and have invites so your blog will be published all across my and other "tribemates" twitter networks.
Blogging is an excellent and cost effective way to market your product directly to those who want it by providing useful information.
I am also offering free business consultations about this very thing if you are interested, and if not, I still hope you find this reply helpful!