face recognition is fail.. lets say bill gates used it. i could steal his phone and then google image a picture of him and hold up upto the phones camera.
same as pickpocketing. i can stand at a distance and use a nice camera with a zoom to take a picture of someones face, and then casually walk upto them to pickpocket them.. then at home show the picture to the stolen phone,
pin numbers and passwords are better because of these reasons
1. its in the persons head so less chance to be directly attacked without close proximity, torture or harder tricks to get to have the access code
2. people lose weight, gain weight get scars, get age wrinkles grow beards.. so that can confuse photo recognition
3. identical twins
4. the effort to press the camera button, adjust the angle, get the focus right, pose and then press snap picture actually takes more time and effort compared to typing a pinnumber or a lengthier password.
and even if that does phase you and you still want it...
This kind of thing should be implemented independently and separately from the blockchain or protocol.
yes it should be a gimmick GUI feature for stuff like multibit or coinbase. not something for core