A little while ago the subject of teaching assignments came up among some colleagues (as it is wont to do). Specifically, we were discussing which PIs had habits of sticking their students on repeated teaching assignments and which PIs didn't have their students teach unless they really wanted to.
I was a little surprised that one of the very well-funded synthesis groups here had an abundance of TAs. One of the students (him/herself a TA) explained it thus: "We don't like to waste money on paying people." The point was that they viewed grant money as primarily for supplies and fancy instrumentation. For instance, they'd shelled out quite a bit of cash recently for some fancy chromatography and microscopy equipment.
It was an interesting perspective, and I'm not sure what to think of the philosophy.
I've seen PIs before who tended to put students on TA quite frequently -- for their entire PhD, in many cases. But those have typically been groups with little-to-no funding, where available grant money wouldn't even cover a meager grad student stipend.
I've also seen well-funded PIs who limit their students to two semesters of teaching (or whatever the departmental requirement happened to be), regardless of external fellowships available to the individuals in their lab. In those cases, a large portion of the available grant money is devoted to stipend/tuition expenses.
So a lab that has plenty of cash that it needs to burn and decides to burn it by buying valuable (but not essential) upgrades--that's different.
It might be a subdiscipline thing. I suspect that "hard" synthesis groups -- methodology and total synthesis -- tend to rely more on a TA culture (perhaps the funding situation is less predictable here?). In contrast, most biologically-oriented organic groups seem to find the funding (from training grants and other sources) to keep their lab RA-based. Additionally, some grants have specifications.
Even so--can personnel costs be considered a "waste"? The word I would suggest is "investment." But it might be because I think a well-trained, happy chemist with decent instrumentation/supplies is more valuable than a slightly nicer MPLC.