Scramjet's may be useful one day. but nobody has actually combined one with another engine in the way that has been done with ramjets. ramjet turbojet combined cycle was done with the SR71 among others. Scramjet may be a possible future upgrade path, but it may also turn out to be no benefit from using it. mach 5 to 6 has been identified as in the ballpark area for optimal speed prior to changing into rocket mode. If you could make a scramjet upgrade that could bring the vehicle from 26km mach 5 to say 50 km and mach 10 you may have something worthwhile. but as it is it is altitude and not speed alone that dictates the swap to internal LOX for the rest of the climb. it just does not pay to go any faster that low in the atmosphere.
The SABRE engine does not operate as a turbojet under any part of the flight. it is a turborocket. the main combustion chamber is a rocket chamber in design. It's even part of the name. Synergistic Air Breathing Rocket Engine. A turbojet uses the combustor exhaust to drive a single shaft turbine. SABRE does not. The SABRE engine uses a closed helium loop to drive the turbo compressors. A case could be made that it is not truly a turborocket either. but it does share quite a lot in common with them. the use of rocket chambers for the main thrust as well as a pre burner to drive the turbo-compressors. but the pre burner is not used in the same manner. it is rather used as a heating source for the helium loop to get enough power out of the system at low speeds. the pre burner is throttled back as the speed increases to accommodate the higher heat flux from the pre cooler.
I generally do not ignore other concepts. but i do take exception to betting the farm on scramjets. they will be too late to the party to make a difference in the race to achieve a viable RLV. 2STO RLV's are the closest competitors, and they have their own issues to contend with. It all depends on their methodology. There is as many flavours as there are space advocates it seems. So until the 2STO advocates get their stuff together and start getting more serious i will be cheering the guys at REL on. They are at this time the most likely bet to get there first.