Although it is hard to quantify if blogging will help your website place better organically, I have to say that from reviewing four different clients in the same industry, blogging can definitely make a difference.
Here is one concrete example, we have four clients in the same state, three of which are located in the same general service area. Three blog, one does not. Monthly I review site placement and prepare placement reports for several of these clients.
One site we have been blogging for since 2006 has garnered top Google placement on many keywords, another site that we designed but just started blogging for is moving up in placement. One site we did not design, but blog for is now placing organically on terms it had not placed on before., and the other fourth site we designed, but is not blogging, placed well initially but now after three month is starting to drop in the search results. Of specific interest is the site for which we did not do the design for is now placing organically, but on the blog posts not the website pages.
From this I can unequivocally say that blogging does help your organic placement. When blogging is teamed with great SEO web design, you can definitely move up in the organic results. Additionally, we have found that when we blog heavily on specific phrases for over 30 days at a time, we can additionally garner placement on these terms. This is particularly important on location or geographic specific terms.
One big note however is to remember that the results you can receive are all based on on-domain blogging versus off-domain blogging. Blogging can be an excellent way to position your website organically, just do it the smart way.