@Sophistifunk cats are absolute killers specialized for human pests. It's like an automated anti-bird turret except it's the targets become ammo. Extremely valuable pest control and probably responsible for saving countless human lives.
To keep things balanced though, people aren't allergic to cats per se, cats merely excrete the mildest of toxins that are harmless to most humans BUT deadly to the weakest humans to keep from poisoning the gene-pool.