That's very antiquated thinking. No teeth are the same. I already said it elsewhere in the comments... But I brush twice a day... Floss... Scrub my tongue/cheeks... Swish and gargle with mouth wash - the whole nine yards. But yet, I always have an issue when I visit the dentist. Just recently I went for my cleaning/checkup last week and I have 2 cavities. My brother, on the other hand, brushes once a day and doesn't floss and never has cavities when he visits the dentist. 

Teeth vary depending on acidity in the mouth, enamel, genetics, etc etc... My father and 2 siblings never get cavities... Me and my mother always do and yes, we practice good dental hygiene habits. It's quite clear genetics aren't in my favor as i've inherited my mother's teeth. I'm fortunate that my parents were able to afford my various dental fillings over the years... But if I weren't so fortunate, i'd be screwed. It should be covered, dental health is exceedingly important.