From an interview with a career SR-71 pilot:
Q: So if I’m understanding the whole startup process is kind of like this space age Model T, where you cranked it just to get the engine up to speed?
A: Yeah. It’s just amazing, and points out a hallmark of the Skunk Works. Don’t waste energy on something you have a solution for. You’ve got a lot of things to worry about already: how to keep the glass from melting at speed, how to keep the engines running at high speeds for long periods, how do you keep the fuel from exploding. If someone had a simple solution to something, then that’s what they did. A very unique, very pragmatic approach. It was also part of the mystique of the thing, this fifty foot green flame shooting out from each engine on startup.