there was none till plot dictated it needed to exist.
On a serious note: I don't think there was a purpose, it's just one of the drawbacks of the jutsu... it can be broken.
it could be as you say that there was no meaning to it until it was needed however.. humor the question.. if there was a reason... what could it be..
imo since i dont see any reason for something that was sealed.. to be unsealed at the cost of the users life.. it could just be a safety measure..
EDIT: on another note..
maybe it was never meant to be a unseeling method.. maybe the mask was meant to bring out the true power of the Reaper by being able to manipulate it using the mask.. orochimaru could have simply used this and cut his own belly causing the reaper to cut her belly in which the souls were being kept..
only orochimaru could have done this since he can easily regenerate and switch boddies
maybe there is a lot more to RDS than we know of