Key to stevens397's case were two factors:

1. Immediate, specific, available use for the purchased miles.

2. Not enough existing miles in the users' accounts.

With the above two conditions, purchasing miles is just another form of currency and can save money, as it did for stevens397.

With all the front-end bonus giveaways these days (not to mention the likely future miles devaluations they will bring), buying miles otherwise is just a bad call.