That is exactly what I was thinking of. E-tickets don't (essily) let you buy a bunch of tickets and keep them for a rainy day. You can keep them in your online account, but you can pretty much guarantee that the day you need to use one is the day you don't have a data connection.
At the very least you can keep 10 pdfs in the Downloads folder of an Android phone (presumably there is an iPhone equivalent) and delete them after use. Someone could probably write an app to mimic m-ticket behaviour using pdfs so you don't accidentally use one twice.