I'd be stealing the idea from Harvest Moon, but the usefulness cats could have perhaps is bringing back rarish items once in awhile. Just right click your cat and "receive gift" every 24 hours, maybe sometimes it would just be a rat, but othertimes something like a river pearl or q 200 bar of metal, or something. Maybe depends on how well you feed your cat.
Or maybe having a cat in your inventory increases your stealth by a certain number of points? You know, what with "cat burglar" and all.
Or alternatively having a cat on the property decreases an intruder's stealth by so-and-so points...
Or maybe all of these things!

Opinions, critiques?