Providing open access to one’s articles results in greater impact as demonstrated by a citation advantage.
Currently, no institution can afford access to all journals and articles (even within the UK many researchers only have access to 2/3 of UK produced publications), though access is a necessary prerequisite for an article to be cited. Open access articles have the advantage of being accessible, and therefore more easily cited. Evidence has shown that articles that are freely available online have a citation advantage when compared with articles published in the same journals that are not freely available. Articles are cited earlier and more often. Additional research has tried to disprove these findings. A good summary of both sides of the argument can be found here.
Antelman, K. (2004). Do Open-Access articles have a greater research impact? College and Research Libraries, 65(5), 372-382. http://eprints.rclis.org/2309/
Davis, P. (2010). Does open access lead to increased readership and citations? A randomized controlled trial of articles published in APS Journals. The Physiologist, 53(6). http://www.the-aps.org/publications/tphys/2010html/December/open_access.htm
Friend, F. (2007). UK access to UK research. Serials: the Journal for the Serials Community, 20(3), 231-234. http://eprints.ucl.ac.uk/4842/
Hajjem, C., Harnad, S. and Gingras, Y. (2005). Ten-year cross-disciplinary comparison of the growth of Open Access and how it increases research citation impact. IEEE Data Engineering Bulletin, 28(4).
Swan, A. (2010). The Open Access citation advantage: Studies and results to date. Technical Report, School of Electronics & Computer Science, University of Southampton. http://eprints.ecs.soton.ac.uk/18516/