What are the benefits (if any) if using "noindex" on your categories in a wordpress. You would think it would be more beneficial to index as much as possible correct? But the Wordpress plugin All in One SEO automatically selects noindex for catagories. Should I leave it as is? Or index them?
It depends on your theme as to whether it's necessary. IMO you definitely want to NOINDEX archives and tags pages (if you use tags) regardless of the theme being used. If your theme displays the entire post on the home, category, archives, and tags pages (instead of simply showing an excerpt) then you should NOINDEX the category pages as well as the archives and tags pages since they will all have a duplicate copy of your post. If your theme supports unique excerpts (instead of simply using the first XXX characters of the post as the excerpt) then I would suggest always writing a unique excerpt instead of using the first XXX characters of the post as the excerpt. And then allow your category pages to be indexed (archives and tags would still be NOINDEX). By doing this your post page will have the only copy of the original post content. The excerpt for each post will be unique and different from the first paragraph or two of the post so the category pages will consist of totally unique content. Another advantage of doing this is that you can embed a link at the end of the excerpt varying the link text a bit rather using the title of the post yet again as the link text or rather than simply using the default "Read more" link text..
NOINDEX prevents your content being indexed thereby avoiding duplicate content. If Google detects duplicate content then your results will be moved to supplemental category. That is why it is good idea to NOINDEX your category, tags pages in wordpress which are basically duplicates of the posts pages and archives pages.