Both of my sister's nieces had bottle rot. Mom couldn't bear to hear them cry at night so they got put down with bottles.
My old assistant (who turned out to be crazy) knew her son had a cavity in his molar but didn't have it addressed because "it's just going to fall out anyway"
But, cavities spread and can infect healthy adult teeth - especially molars which don't come out until later on after many adult teeth are in.
He wound up having such a bad toothache he missed school for two days and had to have an emergency tooth pull
Thankfully my dcp's tend to be okay with taking care of teeth, so I've never noticed an issue here.
My older DD inherited DH's weak enamel and has three silver crowns on her back teeth, BUT it was very clear to the dentist that this was not completely preventable (we were going to a dentist but not a good one who should have noticed this condition) Younger DD has never had a cavity in her life.
There is a huge correlation between dental health and overall health that I can't believe some parents are so lackadaisical about it.