As with all things this is more complicated than would seem at first.
The ferry service normally operates every 20 minutes from 7am to 11pm seven days a week throughout the year. SR operates about 260 days of the year with the first service ex Swanage usually at 10am and the last back from Norden varies between 1600 and 1800. Frequency every 40/80/ 120 minutes. SR is not able to deliver a traditional commuter service and quite frankly the staffing resources required would be immense.
The busiest road in Purbeck* is the A351 from the Bakers Arms Roundabout (A35) to Wareham. Providing a service between Wareham and Swanage takes no traffic off this road unless the passengers first use the mainline service to Wareham. * I have been told but havent been able to verify this is the busiest single carriageway road in Dorset.
The travel time from Bmth to Wareham (mainline) and then Wareham to Swanage (SRC) is uncompetitive with car over the ferry or even the bus. The bus literally takes 1 hour Bmth Rail Station to Swanage Rail Station.
The bus is incredibly busy in the summer on sunny days packed with those entitled to the National Concessionary Bus Pass. Its a hard sell if the train takes longer, some of it is by diesel train, involves a change en route and costs money compared to a quicker, direct (no change) free bus service.
I cant help but feel that SRC would be better off focusing on the heritage steam service between Norden / Corfe Castle and Swanage allowing SWR to operate the service between Wareham and Swanage (or possibly Corfe Castle) with some form of track access fee paid by SWR (to SRC) and some form of revenue share for passengers carried on the common section.
Whoever operates Wareham to Swanage, passengers will expect the standard level of service / facilities provided by mainline TOCS and that will include for example passenger assistance for passengers with needs (mobility impaired etc), delay repay, on train wifi, accessible toilets, appropriately trained staff, departure screens at all stations, TVMs. Would SRC be able to deliver all of this?
Addition: Accept National RailCards, Period Return Tickets (current SRC returns are day returns)