The appeal of Snapchat—and a growing number of companies specializing in ephemeral media—is that once the expiration date has passed, you no longer have to worry about whatever it is you just shared. It’s gone forever. Nobody can dredge it back into public view.
It's not necessary to root your phone to circumvent snapchat or run special software after the fact.
Since you can always use a camera (say on another phone) to take a photograph of the screen and record the image that way, why would anyone consider snapchat or similar services secure or reliable? If you can see it, you can reproduce it, and share it as much as you want via other channels.