Previously it was not recommended to use only the
/%postname%/ for the WordPress permalink structure because it was too resource intensive. But after the update to WordPress 3.3, I see that WordPress has added this as the default option and obviously many serious wp people recommends this structure as seo friendly!
As we were using this permalink structure
/%category%/%postname%/ on our site, it became sometimes difficult to change the post from one category to another after publishing, because the published url got the category fixed with the url. Also in the case of posts with multiple categories the category name was always chosen in alphabetical order in the url, not as the author’s personal preference. I thought lets change our blog posts to this new structure, then it would be easy to maintain/change the categories.