There are several factors at play here. First, most of us want to maintain some sort of AA status. As a result, we will prefer to pay in many cases as award tickets don't earn miles. After that, it's a question of looking for flights that just cost too much for the miles returned. Some examples for us:

We were going to KEF (Iceland) out of Halifax,NS. Going SFO-JFK was reasonable priced, but getting to Halifax was outrageous. So, we bought tickets to JFK and awards to Halifax.

Several years back we flew to SIN and HKG in AA/JL and CX F. Definitely miles

Cheers.