Few people, at least on FT, use miles for AC membership because they find the value per mile too low. There have been threads on this specific point, one of them fairly recent.

The same logic, applied to international upgrades, results in a much higher value per mile. Using that value has also been attacked on the grounds that few of the people who use them would have purchased the upgraded ticket at the usual fare. In my case, at least, that's absolutely correct.

The problem is that there is no single calibration point, AC membership or anything else. The available calibration points vary over a range of at least 20 to 1. Selecting a particular one leaves one open to charges of trying to prove a point by selecting that one and not another. This is a difficult charge to refute, especially if it comes from the IRS and one's tax liability (which taxpayers are understandably motivated to minimize) is at issue.