If you put a sub in a stereo system or any system without a relatively modern surround receiver or processor driving things, you are not likely to have bass management (the RR2150 being the only exception that I am aware of), so there is still some value to retaining the ability to manage bass at the sub (assuming the sub offers the option of bypassing that management).

Some people like to keep the crossover point as low as possible to make maximum use of their speakers and to keep the sub tied to lower frequencies (which are less directional in nature, making the point source of the sub less likely to point itself out).