I didn't think so, but maybe I'm wrong. I would have thought that working as a TA or RA would be a big waste of time while you're trying to do good research and complete your PhD.

In many (most?) cases, the research that the RA is "assisting" on is actually their dissertation research. It would be pretty uncommon to work on a totally unrelated project during your RA time.

Also, many universities require a few semesters of TAing, even if the student is fully funded. It's certainly free labor for the school, but it did help me understand what I was doing....

In Computer Science grad students are often paid by the university to perform research e.g. research fellows (RF) or research assistants (RA). This research helps them complete their PhD but it helps the University in terms of grant funding and prestige/influence. It is often mutually beneficial but it is clearly work.