First, pay by the article means I can easily go through the annual amount I'm willing to spend on "news" in just a few active months. Then I have nothing left for the rest of the year. I don't want to go through the mental effort based on each article and how it _might_ compare to others in the future in order to decide what to read, regardless of having to make that decision before or after I read the article. In other words, it may be a fine article on in it's own right and fully worth 25 cents. But not compared to all the articles I might read in a year. For example, if I have $150 per year for "subscriptions" that means I get 600 (25 cent) articles per year or 20 per month. Is this article worth it?
Second, as mentioned in numerous blogs, paying by the article creates a race to push more articles with just the minimum quality to get the pay click.