My wifes mazda has this and i can pop the flap open with the key. Now if someone really can be bothered to suck fuel from your tank then nothing is going to stop them. Not a locking cap or a locking flap. It would take you less than 2-3 secs to drill a tank.

 

I prefer the locking flap purely from a aesthetic stand point. It looks cleaner when you don't see the little finger hole, IMO.
But if someone really wants to steal your gas, there isn't much you can do.