My personal position on the matter is based on the principle that research publications that are produced using donations, such as funds donated involuntarily through our taxes or time we donate voluntarily when reviewing, should not be turned into private property.
Strategically, limiting the period of time that a paper is kept closed-access turns the question of open access from one of principles ("is charging someone to access work they voluntarily reviewed appropriate?") into a negotiation ("how long is it appropriate for me to try to monetize donated work?"). I would argue that if publishers are successful in shifting the open-access debate from a question of principle into a question of how much they should be allowed to exploit ownership of publications, the research community will lose.
You see, once they get us to agree on an appropriate amount of time to exploit private ownership, we are explicitly indicating that we believe that taking and exploiting private ownership of donated work is appropriate. Each one of us may have a different sense of how much value a publisher deserves to profit from private ownership rights, and so publishers will be able to take back more and more rights over time.
Consider, for example, if we went to a model where we agreed to settle for a model in which all publishers could keep exclusive rights to our work for twelve months. It's unlikely that anyone would express outrage if a publisher (especially a membership-based organization like ACM or IEEE) cited tough economic times and extended private ownership to 18 months. After all, we'd already conceded that private ownership is appropriate for some period of time. In increasing that time period, they can argue either that they understand their publishing costs better than we do or, in the case of a membership-based organization, that the funds are for a good cause that only stingy/selfish members of the community would want to take money away from. A few years later they might ask for two years, and so on. Once we've said that it's morally appropriate to exploit private ownership, and they assert that a small increase in the private ownership period is necessary, it is unlikely that anyone will feel moral outrage or sense that the short increase is worth fighting against. That will make it even easier the next time the period is increased. The equilibrium at which we will find ourselves is the one we've been working so hard to escape.