If you're on a train with a mobile ticket and your phone dies, you're now in breach of the Byelaws. Fine if you're on a Voyager, not so fine if you're on Northern where there aren't any plug sockets.
If the ticket is a flexible one, and you realise you need to travel further/take a different ticketed route/travel in the peak with an off-peak/excess to first class, then you can't without refunding and reissuing, at a cost of £10. With a traditional paper ticket, any decent ticket office can issue these excess fares in a matter of minutes.
If you can't get signal, you can't activate your ticket, ergo you're boarding without a valid ticket.
And of course, it takes the guard about three times as long to verify that a mobile ticket is valid, which on a frequent stopper can really add up.