If you are buying a purebred cat and increasing some market demand to produce more cats this doesn’t apply, but if you’re rescuing cat:

1. You are probably saving the life of that cat.
2. Even if someone else were to adopt that cat, you will be feeding it a more ethically minded diet.

At that point, it really comes down to whether you think a cat should die to protect the lives of its prey animals. I don’t think that, since that line of thought leads to an attempted extermination of all predators.