The argument against such a union is that the students receive significant benefit from their one-on-one relationship with the research professor and this is an integral part of receiving their masters or doctorate degree.

The argument for such a union is that the benefit the students receive is far less than the economic benefit they provide the university for their service and that they have no reasonable alternative if they want to get an advanced degree.

One might ask how this is different from the old days of apprenticeships for the crafts?  If students are required by the university to participate in such research as part of the curriculum, then any stipend they receive is a bonus because the students are also required to attend courses which has an economic cost to the students.  If the students are not required to participate in such research, but are simply offered the opportunity as a means of advancing their education beyond what is normally offered, then any compensation for their time is simply a bonus.

If, however, the university argues that the research projects are not part of the required curriculum, but that the granting of a doctorate is significantly influenced by such participation, then the students are being forced to work despite an official denial that such work is required.  In that case, compensation should be commensurate with what would be paid to an independent contractor and, because there is no negotiation process between the university and student regarding level of compensation, a union would be appropriate as an agent representing the students.