One of the things I've noticed in publishing articles for SEO effect is each website that publishes your article tends to have different title and metatags, so the same article may end up getting ranked on a number of different sites. If a duplicate penalty applied, it would delete the other sites as well.

Probably the biggest reason Ezine articles ranks well is due to page rank and constantly changing content.

The biggest thing I've learned about posting articles is to make sure and use anchor text within the article as well as your resource box. I don't even submit to sites that don't allow anchor text within the article body anymore. Then, when other sites use your article, you'll get the inbound link, even if your specific article page doesn't get ranked.

Hope that helps.