My thought on that is... if you find one at a good price, then yes, buy an extra. But, if you are careful, the impeller in your boat could last for it's entire life.
I put a Skat-Trak swirl in my boat, and the 15 year old impeller that came out of my boat was in such good shape... I kept it. But honestly, the Swirl makes such a big difference, if I damage it, I will just buy a new one.