Points in any program are a continuously devalueing currency. So as escapefromphl noted, only buy them when you have a specific use planned where it will make sense versus paying cash. This isn't Marriott specific. It applies to all points loyalty programs.

For my personal stay patterns, it has essentially never made sense; however, I had also been earning points faster than I could use them from business travel. Even pre-Covid-19 my business travel had been greatly reduced because I moved to a new role at work which requires less travel. So my rate of earning from business stays is down by an order of magnitude. Once I use up the current bankroll of points I have, it may change the math and buying points may make sense in some situations. Bottomline, you need to do your personal math for a specific planned stay each time. How many points you have on hand, your typical rate of earning, your planned upcoming stays, and how much available cash you have to spend at the time will most likely influence the calculations. I.e. it isn't a straight math exercise. At least not for me...

--Jon