Back when US was part of *A, buying miles often made sense even if they weren't on sale, and made a great deal of sense when they were. To use an extreme example, since they placed Russia in the "North Asia" region, it was possible to fly PEK-FRA-SVO in F for 40k RT. @1.3 cpm, this worked out to an all-in price of around $500. More practically, J awards between the US and Asia were 90k miles, or about $1200 all in @1.3 cpm. At around the same time, economy fares were often north of $1500.

However, the economics have changed quite a bit during the course of the past 10 years:
-most significantly, the "pay with miles" trend has been pushing the value of AA/DL/UA miles down close to 1 cpm
*I say "close to" because none of them have figured out a way to apply "pay with miles" to partner redemptions yet
-insofar as my own routes are concerned, revenue fares have dropped a lot in both Y and J

The short of it is that the idea of buying miles doesn't really enter my thought process so much any more. That having been said, there are still exceptions
-if I stumble upon a sweet spot redemption with availability
-using AS miles for CX J/F remains a great deal
-SQ miles can also be useful if you want to fly F on SQ