Think of it this way - at cruise Mach numbers, the engine itself doesn't do much - it's basically there to generate the airflow through the inlet/exhaust. Ramjets don't work at lower Mach numbers - much below ~Mach 2 there simply isn't enough ram compression - hence the need for the turbojet engine. However, the at Mach 3+, you no longer need the turbojet, and pure Ramjets/Scramjets become quite efficient above Mach 3 (really impressive Thrust Specific Fuel Consumption - TSFC). At least in theory, the SR-71 could get better cruise fuel burn if they put some doors in that shutoff the inlet to the turbojet at cruise, let all the airflow bypass the core, and use duct burning as a pure Ramjet. Given the people who designed the SR-71 were no dummies, I assume they determined that the weight/space/complexity of closing off the turbojet inlet wasn't worth the potential TSFC gain. Hence the hybrid turbojet/ramjet system.