The good thing about the AppStore password timing out after 15 minutes is that you can hand your child the ipad/iphone/etc. and not get a surprise $5,000 itunes bill.
That said, it would be nice to have the option of telling the app to cache your credentials.
A short appstore PIN could solve this much more easily.
Now, an SD card with a certificate plus face recognition might be ok ;-)
But for biometrics, keep in mind that biometric systems are currently seen as the most subject to false positives of any authentication system out there with the possible exception of improperly maintained and insufficiently strong passwords.