The manual for a licensed video game I once played claimed that the nacelles are mounted on pylons to separate them from the rest of the ship because they emit hazardous radiation when in use.
According to Memory Alpha, however, each of the Galaxy class’ unusually large nacelles contains a control room for monitoring the warp drive’s operation up close. This implies that it is safe to be not only near but inside the nacelle while the warp drive is running, which nixes the hazardous radiation theory.