This paper (Do Open Access Articles Have a Greater Research Impact?) reports its findings that "freely available articles do have a greater research impact. Shedding light on this category of open access reveals that scholars in diverse disciplines are both adopting open access practices and being rewarded for it."
The findings of this paper have just confirmed what seems to be an obvious argument: the more open the accessibility to articles is, the more they will be used, and thus they ought to have greater impact in research and practice.
An additional question that needs to be addressed in this context is the overall impact of articles published in open access journals. It is quiet possible that articles published in open access journals might be able to shift the focus of a discipline or a field of study because of their wider availability and accessibility.