Well, I did end up getting a whole house (well, whole condo) humidifier two years back to benefit both me and the kitties! If nothing else, it certainly helped with static and their fur, and I think it made the whole house more comfortable for all of us. And, since they're home all day, I think breathing the not-dry-as-a-bone air couldn't have been bad for them.
But are you thinking of a room humidifier or vaporizer, like when a baby has a cold and you want them to break in the moister air?
We were just instructed to get a warm-mist humidifier or vaporizer by our vet, because of two of our cats have upper-respiratory infections with blocked and even bloody noses. I don't yet know if it's helping, but I hope so. And the air in our apartment does feel more pleasant.