I agree with Redyre on every count. I have as many spayed females as I do intact ones. The second I decide they won't be bred or are done breeding they are spayed. I place also all of my puppies on spay/neuter contracts, and all of my rescues are spayed the minute they come home (well obviously not the exact minute, but as soon as I can make the appointment). The reduction in the risk of cancer and Pyometra is reason enough to spay - and the good it does to even out temperament is an added plus.
I'm not trying to be sarcastic with this post, so please don't take it that way....I'm just really shocked that in this day in age that this is still a question. With so much information regarding the health of the animal, the literature that is drilled into people by rescues (I'm one of them), and shelters offering low cost spay/neuter...it just surprises me that someone who is not actively breeding would consider not spaying. I guess that's why we have so many unwanted puppies.
Why could you not spay, personally? Would you rather your dog develop a pus-filled, infected uterus, or cancer, or whatever? Don't think it won't happen to your dog because as dogs as well as any other living thing gets older, everything starts to go. I know it may feel like you might be robbing your dog of something, but it's really for their own good. If it's because of the money, there are lots and lots of places offering low cost speutering. It's just common sense to fix your dog/cat, because ultimately it'll let them live longer.