As a relative newbie in academia, I understand that there is a rather controversial debate raging about the benefits and negatives of Open Access policies. Is it possible to summarize some of the arguments for and against Open Access?
While there's a great deal of complexity in the arguments, the primary arguments basically boil down to a very simple core of inclusion vs. cost.
For open access: Science should be available to all people, including the large number of people who don't have the personal or institutional resources to pay lots of money to buy access to articles.
Against open access: editing and publishing quality materials requires significant resources, and these have to be paid for somehow. If readers don't pay, then somebody else has to (often, but not always the authors or article), and it's not clear if these alternatives will end up making a bigger barrier for low-resource scientists. Moreover, a stable business model hasn't yet been established, and in the meantime lots of scammers have been attracted, who are putting out piles of trash and giving open access a bad name.
There's a lot more here, as noted in the comments, but this is a pretty concise starting point for understanding the arguments for and against.
Proponents of a move to open access argue that this will benefit science and society in general. A report published last April by the UK Wellcome Trust assumes that "the benefits of research are derived principally from access to research results", and therefore that "society as a whole is made worse off if access to scientific research results is restricted".
But even where research is publicly-funded, taxes are generally not paid so that taxpayers can access research results, but rather so that society can benefit from the results of that research ... Publishers claim that 90% of potential readers can access 90% of all available content through national or research libraries, and while this may not be as easy as accessing an article online directly it is certainly possible ... Funding for scientific research also comes from a variety of sources – in some countries such as Australia and New Zealand around 80% of R&D funding comes from the public purse, while in Japan and Switzerland only about 10% is government-funded. It is therefore not necessarily the case that taxpayers fund most scientific research.
Another criticism of open access is that payment for publication could create conflicts of interest and have a negative impact on the perceived neutrality of peer review, as there would be a financial incentive for journals to publish more articles. The importance of the role of peer review does not diminish under an Open Access model, and structures need to be in place to ensure that peer reviewers are not unduly influenced by the needs of their publishers.
In some ways though this argument can apply as much to the current subscription-based system as publishers often justify price increases on the grounds of an increase in the number of journal articles published. This suggests that there are financial advantages for both Open Access and subscription-based publishers in publishing more articles.
All these quotes are from the first section. The later sections of that article delves into more things which are in my opinion quite interesting, such as the argument that open access depresses profits (I'm not convinced it does, but the author apparently is), and depressing profits means less development of information services (such as Scopus and Web of Science) as well as information databases (such as Cell Signaling Gateway). The two points above are, however, by far the most commonly cited arguments for and against open access.