TriMet has an app you can use for buying and using tickets on public transportation here in Portland, Oregon. They were one of the first public transpiration companies to open up their data to mapping services like Google Maps and have tried to be at the forefront of things in this sphere as well.
Despite the convenience and semi-relevance to my work, I don't use the app. I prefer physical tickets. Why?
Simple: you can share them. That's it.
Perhaps there's an inevitable future where we all have smartphone-like devices that would allow me to wirelessly share or buy a ticket for someone. More likely there's a money-less future where we simply don't have to consider such things.
In the meantime, I'll stick with my paper tickets.