Firstly, if you read the Leeds thread, you will see that LBA is not considered the most easily accessible of airports, especially in winter and with its overall weather record.
I would say it is more easily accessible than Manchester for the catchment area it would serve if there are also domestic flights from the likes of Doncaster, Humberside and Teeside. The areas from Bradford to York and Ripon to Leeds all have fairly easy access to LBA.
Secondly, there is actually quite a good train service from Leeds, Huddersfield and York direct to MAN which seem to do pretty good business, so driving across the M62 is not necessarily the only option.
The train service is quite good and I used it a few times but you need to be near the train line to make it work. Otherwise its a change at Leeds. When you have just come off a long haul flight the last thing you need is to be lugging suitcases around and changing trains Most people would rather get a taxi to/from the local airport then a 45 minute flight.
The shuttle has been around a while now, I used it every week last year and there seemed to be more and more people going through to connecting flights. I'm not sure how its been doing lately as I don't use it any more. I would say however that should R3 go ahead the shuttle service connection will be streamlined and tickets probably sold within the long haul ticket enabling flights form say LBA to LAX to be advertised at one price rather than the package being put together separately.
For this to be a success it would need to be opened up to operators other than BA and would suit someone such as Flybe.