With the area having so many of these, there would be rescue help within hours of a tornado. Remember, the local authorities are aware of these and likely know who has them. Moreover, a tornado is a very short event, less than an hour, so it isn't like you will be in there long. After an event rescue squads search every house. Someone yelling inside would be heard.
Jim
+1
I know GJ members love to over think things, but some of the scenarios people are claiming here are a little absurd.
I doubt the door is an airtight seal, so you're not going to suffocate. The door slides and doesn't have any prodtruding handles. So, unless something very heavy lands just right on the door, you should still be able to open it.
Plus, if a tornado is ferocious enough that it destroys your entire home and traps you in the shelter, how well do you think you would have fared without the shelter? Not well, I would guess.
In the very, very rare case where a piece of debris falls just right to trap you in the shelter, you should be rescued pretty quickly. And, if the storm is bad enough to trap you in the shelter, you'll be very glad you had a shelter to get into.