Feed your cats what they will eat. Cat food that meets AAFCO standards is designed to have all the vitamins and minerals your cats need and both dry and canned will give your cats the nutrition they need. You really can feed any combination you want as long as you are feeding a nutritionally balanced cat food so don't worry. Just keep this stuff in mind.
Canned food is better for your cat than dry food because it more closely resembles what your cat will eat in the wild. It contains lots of necessary water and has more protein on a dry matter basis than dry. It also is much lower in carbohydrates and doesn't have nearly as many calories as dry food.
It is ok to feed some dry food if you are on a budget or your cat won't eat enough canned to fulfill his/her nutritional needs. Millions of cats have done fine on it. It's just not as good for your cat as wet food. But that does not mean that your cat can't live a long healthy life on it. I do reccommend not feeding dry exclusively though if your cats are willing to eat some wet.
If your cats prefer wet food over dry food then it is not necessary to feed kibble. If my cat Spotty ate like my cat Rosie I may not even purchase dry food at all. But he picks at his wet and often doesn't finish it and that's where dry food comes in.
As far as urinary tract disease and diabetis are concerned there is a risk if you feed dry food but not all cats who eat dry food will develop these conditions. Diabetis is weight related and fat cats are more likely to develop this. Canned food can help prevent or control this condition because of the reduced amount of calories and carbohydrates. Cats that are prone to Urinary tract crystals should eat wet food. I don't believe that the majority of cats are going to get crystals but because you have no way of knowing if your cat is one of the cats that will be prone to crystals later in life you do run a somewhat higher risk by feeding an exclusive dry diet or a predominantly dry diet.
Dry food does not clean the teeth unless you are feeding a prescription dental diet which unfortunately is made with lower quality ingredients for the expensive price you would be paying for it. There are more premium cat foods for less money. But vets will try to sell you their Hill's T/D. It's up to you if you want to purchase it.
I try not to worry too much. Food is just one aspect of health. The rest is how well they live, love and affection, exercise, comfort and genetics. Genetics has a far greater influence of how long cats will live than diet. Diet just helps to some degree. I just feed my cats as much canned as they are willing to eat and supplement the rest with dry food.
Oh, and in case you didn't know most vets are not nutritional experts. They are influenced by the pet food companies and have short nutrition classes in vet school that are taught by pet food companies. Many vets will advise you to feed dry food because they like the convenience factor and were taught that it's better for your cat's teeth. But there are vets with different opinions and there are vets with nutritional training. Usually they're called holistic veterinarians. Here is a vet who really knows her stuff.www.catinfo.org