In a true home theater system, surround speakers only have to handle the frequency range of the surround sound track, which normally is limited in both the low and frequencies (typically, 10 kHz at the high end). A good book shelf speaker, however, will have frequency response ranging from around 60-80 Hz up to the 18-20 kHz range. Also, true surround speakers are built to distribute the sound more widely, so you will see both dipole and tripole designs that project the sound along the walls of the listening room. Obviously, a bookshelf speaker can be made to function as surround speakers, but their are not optimized for this purpose.
