Requirements for tenure and promotion for faculty include publication in scholarly journals in their field. Usually there is a strong preference for publication in certain journals that are considered more important or high quality.
Impact factor is a proxy for the importance of a journal, which is measured by taking the average number of times an article from that journal in a particular year was cited by other articles in the next two years. There are ways that the impact factor can be "gamed" either intentionally or indirectly, but it is still well-regarded as an indicator of a journal's significance or lack thereof in its discipline.
"Fit" is just as important as significance. Your tenure or promotion review committee will want to see that you are publishing in the area you are "supposed" to be researching in - the area the college hired you to be an expert for them. If you are supposed to be a Middle East history instructor researching the Abassid Caliphate, and you keep publishing in general intellectual history journals, they will not be impressed.
There is also the fact that scholarship is a conversation, and the threads of those conversations are woven through particular publications. If you are researching some controversial idea in heart valve repair, and the other two people in the world who are researching on the same topic are publishing in the Journal of the American Medical Association and the New England Journal of Medicine, you are not going to want to publish your newest finding in Archives of Medicine, even though it's on topic and reputable.
There are also the personal preferences and prejudices of your department and the individuals on your review committee. They may even have a specific list of journals that they would like to see you publish in.
As an individual, you are obligated to take care of yourself. It would not be right to suggest that you risk your entire career and everything that you could learn and share with the world for the sake of throwing your lot in with the Open Access movement. It is not your job to make Open Access work all by yourself - it is your institution's job to support you in making the best scholarly decision - to get your knowledge out there in the vehicle that will get it to other scholars and students who can make something of it. Sometimes that means making sure your library has a subscription to the extremely expensive Journal of the American Medical Association (we do :-)) and sometimes that means making sure your tenure committee values your publication in the Open Access Sociological Science.
The goal isn't to get people to publish in journals that aren't high quality, aren't a good fit, don't have a high impact factor. It's to get researchers to stop not publishing in journals that are perfect for their work, but have a stigma in promotion and tenure decisions because they're unconventional or new.
Faculty decisions regarding publishing their scholarship are arbitrarily constrained as long as tenure and promotion review panels devalue or disregard publication in Open Access journals. The ones in a position to rectify this situation are faculty governing bodies, deans, provosts and institutions' presidents.
The following is an article from College & Research Libraries News that outlines the importance of adjusting policy to recognizing faculty publishing in Open Access journals for the purposes of tenure and promotion.