If you leave it out all the time, you may not see them eating it, either. I know my cats eat, but unless I really look closely at the bowls, I don't really know how much because they don't eat a lot either. Maybe they are eating while you are sleeping? That's when they would be awake anyway. But, if you think they would like a different flavour, give it a try! A little variety never hurt anyone! Just make sure when you introduce a new food of any kind, do it gradually and mix it with the previous food, so it doens't upset their stomachs.

Good luck!!