I have purchased points in the past to be able to redeem a "travel package" reward. The main reason that I went ahead and purchased the points was so I could be sure to get the flight that I wanted with the airline miles.
While it was nice to have the points available when I wanted them I doubt that I would purchase points to just have spares. That being said personally it is not likely I would have a emergency that required me using my Marriott points and could not be paid for in cash.