probably the best reason to get a sub is to unload the lower frequencies off yor main speakers. you do this not to save the speakers, but to save the receiver.
low-end grunt (bass) sucks a lot of power from your receiver that could be used for much more useful purposes. It will be pushing hard to drive the mains and may not sound as good or last as long.
even some of the sub-$ 200 Yamaha or Velodyne units will give you a decent, maybe not spectacular, sound, but they will definitely help out the "big picture" for the receiver.