If your blog is not indexed by search engines, then it does not exists on the web. For any blog, search engines are like god. The amount of traffic generated by search engines is the key factor that decides success or failure of any blog.
The basic WordPress blog setup is not well optimised for search engines. If you look at the problems of basic setup: URLs are not meaningful, titles are not well formatted and search engines does not know where is the content is & how to index them.