with out reading the entire thread, I do have my older cat taken in every 6-8 months to have her teeth cleaned.
I dont think it is that important for younger cats.. I mean.. cats live LONG healthy lives in the wild, with out human "assistance". Espically with out tooth brushes..
Indoor cats are even less likely to be exposed to some of the things that outdoor cats are.. so in a way, I think they are safer .
My cat is 7. Not 'old' but it is time to start paying attention to her health and do a little more to ensure basic health and comfort. Just like humans, cat's gums can become infected, teeth can become sensative, cavities can begin to grow ... etc.
A simple trip twice a year to have her teeth cleaned can help guard against those things. But I am not sure I agree that it is 100% necessary for all cats..
I have had some Vets try to get me to bring my cat in before she was even 4 years old for monthly teeth cleaning..