Is there a true unsatisfied demand for through ticketing, are people that reluctant to use bus links because they have to buy a separate ticket, or is this just all a bit gimmicky...
When visiting an unfamilar area, I find that it is rather more difficult to find times for a multi-modal journey compared to just a rail journey. Yes, there is the Traveline website but I don't find it nearly as easy to use as the 'Mixing Deck' rail journey planners (like FirstGWR's). In the rare event that I want to get somewhere away from the rail network while on holiday the steps I have followed go something like this:
1. check which county my destination is in
2. go to the appropriate local authority's website and look for bus information (with some councils, like Powys, I draw a blank), where the journey crosses a county boundry check both authorities' websites. If I'm very lucky they will have route maps and timetables, if not and they only have a list of route numbers then
3. hope that the operator of the route is listed and check the operators' website for timetables and route maps, potentially multiple operators serving an area for the maps
4. spend hours looking at the timetables and working out whether the journey I want to make is possible or if the connections don't work, or the village I wanted to get to is not served by regular buses (in which case visiting the place gets dropped from my holiday plans, or a taxi is needed)
5. have no local knowledge of which routes are punctual and fail to allow enough time for a connection which is then missed throwing the plan out of the window anyway
Not that I've tried using it, but the 'Minehead bus' destination on the FirstGWR 'Mixing Deck' looks far more straightforward than the above, the timetable for the buses is show along with the trains. That, plus the things Trowres listed above, mean this 'virtual railway' idea, depending on how it is implemented, might make a big difference.
The through contract mentioned by Trowres in itself could be rather important. Currently, missing a connecting bus because the train is late means having to pay for a taxi yourself to complete the journey, and if your bus is late on the way to the station you are stuck; but if a train-to-train connection is missed due to a train being late then (unless there is another train along shortly) the railway is supposed to pay the taxi fare for you.