Spaying is for health reasons beyond just being able to have babies. Cancer is one of the worst offenders of taking our feline companions from us. Spaying reduces the risk of some of those cancers significantly to completely.
Mammary cancer is the 3rd most common cancer in cats and is most common in female cats that have not been spayed.
Spaying removes any possibility of cancer of the ovaries or uterus.
Pyometra, a possibly deadly uterine infection, can't happen without a uterus.
We are currently working to get the cushings disease under control in a 14 year old dog we rescued in order to get her spayed. My SIL (Maxi's vet) just doesn't think it is worth the risk to leave her not spayed even though she is so old she is not likely to be parenting any pups.
I wouldn't put it off. I know surgery and anesthesia of any kind on our babies is scary, especially the older ones, but some of the stuff you can risk is pretty scary too.