The decision many departments make to move grads to hourly summer positions is, I think, made mostly because departments avoid paying for nine credits of tuition waiver that way, which is a totally reasonable desire. The current problems that I see are (a) lack of subsidized health insurance and (b) lack of contract protections. (The fee differential doesn’t currently apply to summer anyway.) The OSU bargaining team always makes a big deal about summer being different than the rest of the year, and so far we haven’t prioritized working on summer stuff. That could very well change in the future – I imagine if the nine-credit requirement were changed, departments would be much more amenable to keeping grads on assistantships, unless there is some other piece I’m missing.