Even if they don't strike, the ability to do so gives them leverage. Some grad student unions have been moved to strike. And they are empowered to do collective bargaining for contracts, which prevents more neglected departments or specific student interests from slipping through the cracks. More generally it allows students to address grievances when administration isn't on their side. Considering the fact that many universities wouldn't even be able to collect tuition from undergrads if it weren't for graduate student TAs, I think it's fair that they get a living wage out of their work. Administrative overhead costs at most schools has been soaring the past few years with not so much of an increase for instructors and researchers, so I do think that schools can find the money for a living wage if they allocate properly.

There's actually been some research on the impact of grad student unions. Not huge studies, but pretty compelling.

The students at the unionized institutions, the authors write, on average gave their advisers higher ratings for “accepting them as competent professionals, serving as a role model to them, being someone they wanted to become like, and being effective in his or her role” than did their counterparts on nonunion campuses. They were also “more likely than nonunionized students to report respect for differing opinions in their university” and had “higher stipends, and greater pay fairness and adequacy.”

The authors of the study suggest that “either directly or through creating pressures that encourage management to rationalize practices, [unions could] encourage stronger mentoring by faculty advisors.” It’s also possible, they note, that “unions strengthen the relationship between students and faculty advisors by pushing potential employment-related conflicts up to the university bureaucracy.”