My debit card, for example, has "paywave" short range payment support. So anybody who has my card can go around making small purchases, no PIN, no signature needed. I'm fine with this because the convenience far outweighs the security concern.
With the iPhone an attacker who replicates your fingerprint can make purchases to your iTunes account using your phone. They can't purchase to a different account, they can't purchase to a different device. In that sense, requiring a valid fingerprint is more than secure enough — even if faked it's not going to do much damage.
Creating a fake print that can fool the scanner is so much harder than stealing someone's debit/credit card. It's also so much less damaging to the victim (making purchases on their iTunes account vs. making any arbitrary purchase).
I think the balance between security and convenience for this technology is more than reasonable.