A. Because we want to take advantage of the resource box to get a straight one way backlink from the article page and also in some cases, get traffic.
Q. What else do we expect?
A. We expect that the article would be picked up by bloggers looking for content and they would put our article on their blog without changing it in anyway and keeping intact the resource box to credit us and thus we get a backlink and traffic sometimes from their blog too.
But this isn't true always. Naive and new bloggers forget to copy the resource box sometimes. And sometimes they copy the entire resource box without realizing that the link behind the anchor text isn't copied. This way we don't get the deserved backlink.
Q. But why this doesn't/won't work anymore?
A. Since search engines hate duplicate content, they would hate to see the same article on different pages - on our article directory page, on the blogs of the bloggers who have picked up our articles.
Q. How will it harm me/us?
A. In many ways!
Consider this example: Suppose I write a good article and put it on an article directory with a link back to "my money site". The article goes live. A new blogger super excited with the hopes of making quick bucks online looking for some content to put up on his blog makes his way to my article and decides to copy and paste it on his blog with resource box intact.
But now my article with intact resource box pointing to "my money site", is on two web pages, i.e., my article directory page and the blog of the blogger. And these pages are linking to my money site, with the same anchor text, same content (article) and guess what, search engines don't like this. Search engines would think "this dude is trying to fool me by posting the same article on multiple blogs, websites etc and trying to get a backlink link from there for higher rankings. i dont like this dude and i will slam the site in the anchor text".
And so the search engine slams the site in anchor text, i.e., my money site.
The above scenario would have been perfect for getting backlinks if duplicate content wasn't an issue with search engines.
I will not be posting onto article directories for backlinks and even for traffic as few of my sites were slammed when they were picked up by bloggers. And my article directory page, my money site in the anchor text in the resources box, all were slammed by search engines for my keywords I used to rank for.
Well this may not apply, if you have big authority sites but my few niche sites obviously were slammed by this. You may think I am talking totally non sense but if you have lately lost your ranking on search engines, then this could be a reason.
P.S. Although, this is not the fault of article directories nor the innocent bloggers who were just looking for some content. I just wish search engines were smarter.