Are publishers actually helpful for knowledge dissemination? Or are scholars better off DIY?

Melissa Terras' article in the Journal of Digital Humanities, The Impact of Social Media on the Dissemination of Research: Results of an Experiment found that tweeting her author's final version of papers in the institutional repository resulted in a significant increase in downloads. To me, this raises the question: is the role of publishers in disseminating and marketing research articles rapidly becoming obsolete? This would be an interesting question for altmetrics researchers to explore!