The idea is to allow for an otherwise high-efficiency thermal rocket to 'switch gears' by injecting oxidiser into its exhaust stream to sacrifice exhaust velocity in exchange for extra thrust. CDE's focus on combat makes pertinent demands for short bursts of high thrust, and in the pre-release blog qswitched seemed to express a feeling that capital ship dodging would be fairly important and discussed VASIMR briefly, so it seems odd that this wasn't implmented given that it is ostensibly relatively simple. That said, the concept doesn't seem to be well known; perhaps it has been missed. Though it is discussed specifically with an NTR, namely the LANTR illustrated below, presumably it is feasible in resistojets too.
IMO the high density of LOX means that for a lot of in-game ships it would be ideal to run in afterburner all the time.
We dig deep down into the very building blocks of life, seeking to twist them to our will. We destroy mass itself to turn its energy to our ends. The speed and intelligence of our machines grows without bound. But even the power of gods isn't enough. Ever hungry, we turn our eyes to the stars...
I sure hope there's someone else more advanced out there. If we're the first, the universe is screwed.