I understand that, apples to apples, a cat back exhaust is the better choice.
However, most exhausts (cat or axle back) don't add a lot of HP and from the axle backs that are available for the F30, they do change the sound. Not to mention, the cat back does normally cost more than an axle back.
Whether its worth the cost can be debated, but I don't see it as a "stupid purchase"
The aFe is $194 more then the BMW axle-back and the Magnaflow is $56 less, so that's why I call it a stupid purchase.