If the goal is to get back links, then every article you submit will count as a back link if its relevant. The more places its submitted to, the more back links you have.

Now its true the lower ranked sites won’t get you the link power the higher ones do, but they post faster and so you get their benefit faster, you get your recognition from Google from them, and then when the higher directories start posting your article, you get that too.

Kind of a gradual increase in site value

I find the best results come from hand submitting articles to only the top directories — EzineArticles being my number 1.