How about security? How does it protect other uses from controlling your device?
Really? And your phone is on? With the screen visible for them to see what they are interacting with. And as they are fumbling around in your personal space inches from your hand or pocket, no one notices.
Get real! If true you should be more worried about pickpockets than some random gestures.
You know, check for what's the foreground process, notice it's a password manager or bank application (or even a lock screen would do, who knows, maybe user had reused the same PIN elsewhere), blast the speakers with ultrasound and wait for the typing-like movement patterns.