I'm a vet tech working for a doctor who has been treating cats exclusively for almost 20 years. I see a good bit of misinformation on here. I give this speech too often, so I'll keep it as brief as possible.
Canned food is much closer to what cats are naturally designed to eat.
Cheap can food will make their stool stink. You can grind up chicken beaks and feet and still call it chicken protein. That doesn't mean their bodies are able to utilize it. The left over "junk" is what you smell.
Dry food needs way more carbohydrates (to hold the pellet together) than a cat needs.
Dry food is fattening b/c it is dense in calories from carbs and doesn't provide enough protein to satiate.
Dry food, in general, does nothing for dental health. There are a limited number of dental diets available that we have seen actually work, but most dry food pellets are baked so hard that it is equivalent to us cracking ice.
Canned food is much higher in protein, so it satiates better than dry. It also has much higher water content. Cats are descendent from desert creatures, and so natuarlly rely on thier prey to provide hydration, and often do not drink enough.
B/c of the higher protein level and water content, we always recommend can food, especially when a cat needs to lose weight. THIS WORKS, I kid you not.
Dry food was invented for human convenience. It is not meant to be an ideal diet for a cat.
Cats do get bored with their food and should be introduced to a variety of quality foods. What would you do if someone handed you a bag of crackers and said "this has been fortified with everything you'll ever need - only eat this for the rest of your life" ? Laugh in their face? So why do the same to your cat? No one company knows everything there is to know about feline nutrition.
Other notes:
Yes, they seem to be doing fine on dry food, but they could be doing much better.
Don't free feed. Housecats will eat out of boredom, just like us. Small meals throughout the day is more ideal. We recommend 1/4 cup dry twice a day OR 1/2 of a 5 oz can twice a day OR one 3 oz can twice a day. If feeding can and dry, only 1/8 cup of dry once or twice a day.
It is NOT normal for cats to vomit. If your cat vomits regularly, or every time they eat a certain thing, there is something wrong. If your vet says otherwise, find a new vet.
Dry food should not sit out longer than 24 hours. It goes stale. Would you eat a cracker that had been sitting out on the table for two days?
Don't add new dry food on top of old dry food. It promotes growth of bacteria.
Feed your animals out of fresh, clean bowls every day. Or if you're lazy, like me, just use a new paper plate every day. Cats require fat in their diet. To get that into a dry food, they spray a coat of fat on the outside of the dry pellets. Feel the inside of the food bowl - it's slick. That's the fat. The fat can become rancid.
If your cat has itchy ears, it's probably NOT ear mites. More than likely, it's food allergies.
Yes, you need to scoop the litter box at least once every day. You don't like to use unflushed toilets. Why should your cat? By the way, feline behavior experts recommend you have one box per cat plus one extra.
Sorry if this isn't all prettied up as suggestions, but it's just facts. And for the record: I have two cats and three litter boxes. One cat is diabetic and one has food allergies. I feed a combination of dry and canned that meet their special dietary needs. Hope this helps someone.