The problem is that if the low quality render differs in any appreciable way from the high quality one, there will be flickering. So the low quality renders have to be extremely similar to the high quality ones, in which case why not just use the low-quality ones all the time?

Though come to think of it, there is one example that almost does what you describe. Dynamic resolution scaling has some artifacts, but is being used in some games (and is notably used in Dead Rising 3). Though one has to decide before the frame is rendered what resolution to use, so you still get frames that take longer than 16.7ms or whatever your target is.

One could do something similar with render quality, but it has the same drawback that you have to decide beforehand what quality you want to use. One would also have to ramp up and down the quality slowly, which is difficult as the complexity of a given scene can vary wildly over the space of a single second. It also wouldn't help with spikes in rendering time.