I think Elsevier is a good position to act as a kingpin in the math world when it comes to the academic publishing racket. I'm not sure which publisher is worse than others, but I do think it's more likely that success will come against a particular target (more researchers will take the pledge), and that things would change with all publishers as a result. Elsevier has a long list of well-respected math journals, so a victory with them could be most meaningful in the eyes of the mathematicians who choose where to submit their papers.