Clearly some people on this thread don't understand what's going on here. So let me break it down....
It's not about "it's not a real friend if they invade your privacy".
It's not about "why not just lock the phone itself".
Maybe you have a family member, girlfriend, wife, or CHILD you want to be allowed to use your phone. This requires unlocking the phone itself.
Buuuut, you don't want them snooping in your email or for the dense folks who don't get it, you don't want a CHILD deleting your messages accidentally, replying inadvertently to a co-worker via email etc.
Or let's be real, you don't want your GF or wife to see the chic you've been bangin's nude photo in your phone.
So you want to FINGERPRINT LOCK specific apps, like TEXT, EMAIL, SKYPE, TANGO, FRING, ETC! That way your "guest" can use your phone, but can't use any other apps without your finger!
I'd really like to see phones with Windows-like login profiles. So I can have my profile, ADMIN, and my wife can have hers GUEST.
Anyway...I second/triple/quadruple the idea of a fingerprint reader app lock. Right now I use AppProtector, and I'm looking into a new app called simply LOCK or (App) Lock.