Sometimes it is very necessary! Just like people some cats need it more than others. My dear, departed Siamese Manx became diabetic at age 12. Turns out it was out of control gum disease. Cleaning his teeth every 8-9 months and a diet change to grain free and we kept him off insulin until he left us at age 18!
Yes, you can brush but that is not going to get any problems below the gum line.