There is another scenario (which actually was the case for me a quite often during the last couple of weeks):
Only Best Flex available (period for adv purchase booking is over)
--> +1000 points - rates only 2€/5€/10€ more in many cases
Since these points count (as opposed to Plat/Amb Bonus) for the status I sometimes book them now.
IMO it is not (any more) about buying points only but rather about buying status