Who would use it? Those who want fast (and turn up and go) use the 4-6tph train service between Shipley and Leeds.
Those who want cheaper get the bus. An express would have to attract a lot of new passengers to be worth running.
People who would prefer the bus over the train, but to get them there faster. This has already happened on the 760 route by running fast between Keighley and Crossflatts, and again from Bingley town centre to Nab Wood. These buses seem well loaded and would take pressure off some of the Airedale Line trains if they had more of an express route on one of its journeys each hour (maybe an extra journey).
Not everyone wants to use a train. Don't forget how far the bus and rail stations are in Leeds. What about people connecting to other buses in Leeds, or National Express/Megabus?
Express buses operate to and from Nottingham and Derby from outlying places. I won't include the very successful Red Arrow services as they are inter-city so have a bigger population to serve, and Derby station is not exactly central, but Long Eaton and Bingham have both had express services to Nottingham. Both towns, especially Long Eaton are reasonably well served by rail.
Buses in West Yorkshire are slow on the whole, but look at the X6. A faster bus between Leeds and Bradford, despite 6 trains an hour between the two. Generally well loaded. We need more! Wakefield to Leeds on the 110 is always busy, despite 8 trains an hour taking as little as 13 mins. Wakefield could definitely do with a non-stop bus to Leeds running via the A650/M1/M621. Places like Halifax and Huddersfield could do with similar.
With proper branding and marketing, express services in West Yorkshire could be a commercial success. Like I said, not everyone wants train travel, especially in its current state up here (yes, it will improve). The 36 to Harrogate/Ripon, another success story. I don't think passengers from the far flung corners of West Yorkshire would object to faster bus services.