I see it more as a feature than a flaw, and I was pleasantly surprised when I found it! You miss a call and can return it without having to unlock the phone (not that different from the emergency call feature). In this case, you can only return a call, and I doubt someone that took your phone would want to call someone you know... Just a thought.
Now, yes, it does lessen the security of the thing a little, but it's always a tradeoff between security and usability. Personally, I like it, but I can understand if people don't... This one just boils down to perspective.
It's just another reason they need to give you more options and access to customize this stuff the way you want it to work... not just the way Apple allows it. Oh well...