When I was a grad student, my advisor had to decide whether to give a Research Assistantship to me or to another grad student. In an alarming moment of frankness, he told me that, although I had earned this RA, he was going to give it to the other student because he knew that I would get a lot of research done whether I was a TA or RA, whereas this other student would only make research progress if he were an RA.
I have thought about that incident from time to time over the years. When using precious RA money, sometimes you have to make a difficult decision about when and on whom to spend the money. Grants have a finite lifetime (even with a no-cost extension or two), and it is essential that the funds be optimized. Most PI's have had experiences in which RA funds did not result in much research activity*, even when students were not also taking classes or preparing for preliminary exams. Such experiences should be avoided if at all possible.
* Example: I once paid a student an RA during a term in which the ratio of wedding planning to research was something like 20 : 1. I am rounding up the second number to create a positive integer value.
One way to stretch grant funds is to give a student a 50/50 TA/RA split appointment in a particular semester. Reasons for a split appointment can include financial considerations related to grant budgets or a student's wish to TA a particular class (but not to teach as much as a full TA appointment).
I have found that, in general, motivated and efficient students will make excellent progress with their research whether they are a TA or not (or a split TA/RA), and less motivated and inefficient students will make no research progress while on a TA appointment and some progress while on an RA appointment. This is something my former advisor knew decades ago.
But would I make the same decision he did? Now that I've been advising students for a number of years, I can understand his decision more than I did at the time, and I also now understand why he told me the basis for his decision. I think that I would be very tempted to make the same decision he did, but in the end, I don't think I would.
6 months ago