The main benefit of Clipper, though, is one stored-value card that works throughout the region: BART (commuter rail, peninsula and east bay), CalTrain (commuter

The main benefit of Clipper, though, is one stored-value card that works throughout the region: BART (commuter rail, peninsula and east bay), CalTrain (commuter rail, peninsuala/south-bay), MUNI (SF buses and rail), AC Transit (east bay buses), SamTrans (peninsula buses), GGTransit (north bay buses/ferries), etc. A tap-in card that's locked to certain prepurchased trips wouldn't be Clipper and might not be of much interest to either commuters or CalTrain operators. A ticket-book works fine for that predictable travel.