They are probably just there to make it harder to find the Reaper death Seal mask. I doubt that there would be that many masks left out in the open if each one corresponded to a powerful jutsu. The first thing to release the RDS would be to know about the mask in the first place. Then, the person would have to find its location, and then would need to be able to find the right mask. Finally, the releasing of the seal requires a living sacrifice.
These should all be difficult, or Orochimaru would have just went and released the seal before Sasuke killed him. In doing so, he would have gotten back two of his Edo Tensei bodies and access to his previous jutsus. Plus, he would have been able to add the remaining two Hokage to his arsenal. I think he knew something about the location of the masks (or at least which one was the correct one) and Kabuto learned the location later (with Orochimaru inside him). This would also explain why Kabuto didn't add the Hokage to his arsenal. Just imagine the War Arc if Hashirama and Madara were fighting on the same side. I think that if somebody tries to use the wrong mask, they are in turn sealed, too.
