I certainly see nothing wrong with printed e-tickets for walk on journeys. The only possible fraud is someone using the ticket for two return journeys in a day, and hardly anyone wants to do that.
If every member of railway staff that deals with tickets is given suitable hardware to read barcodes and then send back data immediately to block it (quick enough so it doesn't scan the identical copy the next passenger has in the same group, let alone on a later train) then, fine.
As it is, there are usually restricted routes for which these are valid and often tied to a particular train or seat. Even in Sweden, they've not always scanned the barcode but simply crossed our name off an old-school list on paper. Of course, the X2000 service is reservation only so it's a bit easier to do.
And if staff have connected terminals, they could check if tickets had been collected. If you didn't have time, you could even potentially get the tickets printed on the train at the first opportunity.
But we don't have that yet, so it's not possible. And I could very easily print off a ticket today, give it to my wife, then claim to have had trouble getting my ticket and show a booking reference. Heck, I could even fake a booking reference by just loading an email into any editor and changing the code. After all, the member of staff CANNOT check it as it stands.
I wouldn't do that. You wouldn't do that. But if it was possible to be allowed travel, PLENTY of people WOULD do that.
The issue is obviously cost, and ultimately we're working towards smartcards and that solves a lot of the issues we regularly discuss here. So do you bother with the printed barcodes, or m-tickets on a phone that might have a flat battery when needed, or just skip on to the next generation and hopefully get that right?
Perhaps in the short term, given paper tickets aren't going anywhere overnight even with smartcards, you could have more, smaller, machines that do nothing but print off pre-booked tickets? Perhaps even include scanners (ala petrol pumps for loyalty cards, or Amazon lockers for goods) to scan a code on your printed reservation or on-screen - to save having to enter the reference code. This will then enable people to collect tickets quicker, along with locking the tray until every ticket is collected (or flashing red while printing, and green when ready).