psyxologos, Jon D, xenon
As I said its not 100% fool proof, but it's not 'that simple' to just rip off the plate or unscrew the screws (or tamper proof)... The person trying to steal your car is trying to make this quick, with minimum fuss.
Most likely the potential thief has broken your window glass, and then slides his hand by the inside of the door to access the port and slot in his OBD cable. Opening screws with a screw driver/ ripping off a plate will require a lot more movement/ intrusion and the car alarm is likely to sound.
As I said, not a perfect solution... In fact I doubt there is any perfect solution. If your car is being stolen 'to order' then it's likely to be stolen regardless of what you do.
This is possibly a deterrent... It's better than not having anything at all.