Daniel Oppenheimer with colleagues Connor Diemand-Yauman and Erikka Vaughan found that students presented with material written in hard to read font (grey, 12-point Comic sans) recalled 14% more facts than students given material in pure-black, 16-point Arial.
Of course, the subjects were required to read the material in those tests. It is less clear how you might use this in a marketing context where the goal is usually to remove any potential point of resistance to your audience receiving your message.
The knee-jerk reaction is to assume this means that there is nothing of use to marketers here. However, I wonder if there is a way to use this in a subtle way. For example, what would be the impact of using a slightly less clear font for highlights or summary points? Or if those important points are in the same font but a colour that offers less contrast to the main text: could such a point be both more appealing (drawing the eye) but still require the fractionally greater degree of concentration that makes the fact register more readily retained?