I think the main reason the various TOCs▸ only offer m-tickets, e-tickets or various printouts for Advance fares is that they can send info to the specific booked train's guard that allows him to know how many to expect to check. Same as SWT▸ do with their 'megatrain' booking reference numbers.
If you allowed an m-ticket for any walk up fare, every guard on a number of possible trains would all need the same information, and there'd also need to be a watertight system to avoid duplication...
Yes, you'd need some system to avoid people using the same ticket on multiple journeys. Presumably Swedish railways have some way of doing this. If not, then as paper tickets are supposedly to be phased out within the next ten years (thread passim), that implies that fares not linked to specific trains are also to be phased out, which would make rather a mess on certain lines where there's nothing else.