Adding a blog or forum help in SERPS
Content is content. If the blog content is good and well integrated with the rest of the site it can’t hurt.
I found that a blog named for a keyword phrase and linked from all pages of my site helped in boosting the serps for that keyword.
We have seen an increase when we added blogs and forums to our site.
Content is king, and a blog will improve your long tail searches, an active forum even more so.