Surely a card can be issued for fans who wish it, with the right technology built into it. Dare I say it a Boro Card. My phone cover actually has a slot for such Cards. That way the Club are more fully inclusive. I think the supporters club should lobby for a physical card option to avaialble, if they haven't already.
Ref ticket sharing I would say it coud become more common with phones than a physical card, because the cards are coloured differently aren't they? - so its not difficult to see an adult using a childs card or ask them to produce the card.and its quickly visible there is an issue. If some is determined enought they could use someone elses phone and no one would know or it may be possible to move the ticket from device to device. A club official can't tell without going through someones phone which could cause a lot fo friction. As said there could be light on turnstiles, but that is there now and it needs a steward say for every 2/3 turnstiles. I have never seen anyone stopped, so the abuse can't be high, but does need to be checked for.
My guess is the new system is primarily for :
fans who want everything on their device and thus don't have to carry a seperate card or print off a ticket
saving the club producing and distributing, as many physical cards and tickets.
which I can understand and I am not against.