There is a "combined cycle" engine that is purported to be MORE efficient that basic diesel. Such an engine uses both natgas and diesel. Double injectors on each cylinder. The natgas injection provides most of the BTU's for driving, and a very small amount of diesel injected retains the "compression/ignition" of the diesel and obviates the need for a spark ignition source.
I suspect that for future "big rigs" that such engines will become the drive option of choice.
Two high pressure injectors is too much added complexity over just firing with plugs. Most of the dual mode units I have seen online (never have seen a dual mode in person), are conversions that just had the diesel around already, so why rip it out.
Turbochargers and direct CNG injection with spark ignition is the way to go, IMO. Most diesel designs already take a turbo into account, and most modern ones even have wastegates that can be computer controlled (or already are).
I was jealous of a friend in the Silicon Valley area in california. He bought a well-used police CrownVic with CNG conversion. It needed a valve job on purchase (did not have the hard face valves that last better with CNG), but now he pays a pittance for fuel costs, and that area has plenty of CNG stations.