I have two clients that have something like this. They are buried right behind their houses, in their back yards. Other than opening it up when they first moved in, neither of them has ever gone into it when a Tornado was in the area. It's usually just after dark, storming out with heavy rain, and tons of lightning in the area.
We thought about building a tornado shelter in our garage, but now we're going to convert a room into a second master bedroom closet and build it according to FEMA plans for tornados. It's just one of those things that can't happen until ten other things get done first.
In my opinion, if it's easy to get to, it will be used.
I've seen several new homes that have the master closet built as a Safe Room. Two doors to get into it. A normal door that swings out, and a massive Safe Door that swings in. From what I've been told, venting those rooms has been the biggest issue with the City, and passing inspection. They are still trying to figure this out and it seems to change from building to building.