This is something I have thought about as well. I cannot recall, but how do ships move about withing a star system? Do they warp from planet to planet, or travel at impulse? If they are warping from planet to planet, then you can assume that impulse speed must be relatively slow, and not very useful for traveling any great distance. In this case, Impulse Drives could be viewed as both a back-up (very limited) to Warp Drive, and as short range maneuvering thrusters for docking and such. Otherwise, we can assume that traveling at Warp is "expensive", meaning that traveling between planets would be easier done with an Impulse Drive, rather than wasting Dilithium or whatever to travel a relatively short distance. If this was the case, Impulse Drives would be capable of something like 25/50/75 percent of c
, allowing travel from one end of a solar system to other within a day.