Terry Tao made a good point when he joined the boycott of Elsevier journals. He said he would stop submitting to their journals, and stop doing editorial work for them, but declined to stop reviewing papers.

The reason he gave is that while he is at a stage in his career where there is no pressure to publish in any particular set of journals and so can exercise discretion in where he publishes, he recognizes that some mathematicians, for reasons beyond their control, are unable to do so, and he does not wish to penalize them.

For fields where many of the high impact journals are not yet open access, I think Tao's approach makes sense. Publishing in high impact journals greatly helps the careers of young researchers.