First of all, they can fall out. Let's take as read the nutritional problems and aesthetic considerations of having no teeth: I'm more interested in the second possibility: they can rot. That hurts. It looks bad, smells bad, keeps you from eating nice crunchy things like vegetables, keeps you from smiling as widely as you otherwise might. It can also leave you with what's known in the medical biz as a flaming clusterfuck of problems. To wit:
Let's say you're a sixteen-year-old girl who's had minimal access to medical care and no access to fluoridated water all your life. You come to me with a history of a heart valve replacement, complaining now of constant severe headaches and dizziness. What do these two things have in common?
Believe it or not, rotten teeth.
See, if you have really, *really* bad teeth, you're setting yourself up for bacterial infections that can lodge in your heart valves. If the infection in your heart gets bad enough, you'll end up having to have one or more valves replaced because they'll be so damaged that they won't work correctly. *That* means you'll have to be on warfarin, a blood thinner, for the rest of your life so that you don't get clots in the new valves that could travel to your brain and cause you to stroke.
The trouble is, if you don't have access to comprehensive medical care, the original problem (teeth) won't be taken care of. That'll lead to bacterial infections that travel to other places. One of those places might be one or more of the arteries that feed your brain. That, my friends, is known as a mycotic aneurysm, and it is no fun at all.
Further, since you're already on blood thinners to take care of the clotting possibility, that aneurysm (which is nothing more than a stretched-out, thin spot in an artery) will probably leak blood now and then, damaging your brain.
Oh, and you're four months pregnant.
So let's review the bidding: no medical care, no dental care (to speak of), rotten teeth. Rotten teeth lead to an infection lodging in a heart valve that goes undiagnosed and untreated until the damage is so extensive that the valve has to be replaced. Blood thinners to reduce the risk of clotting from the new heart valve, but (again, poor medical care) the levels of drug in your bloodstream aren't monitored, so your blood gets too thin. Meanwhile, another infection has lodged in your brain and stretched out an artery, which then bleeds a bit now and then and makes you miserable. And warfarin and early pregnancy do not, in the least little bit, mix.
And you're sixteen.
I have never, in all the years I've been at Sunnydale General, seen as many consults on one patient as I saw on this one. We had dental, OB, cardiology, internal med, neurosurgery, neurology, pediatrics...and those were just the medical consults. Her chart was overflowing and she hadn't even been there a week.
The end result was this: we took her off warfarin and put her on a heparin drip. Later, when she was ready for surgery, we took her *off* the heparin and fixed her aneurysm. She miscarried her pregnancy and had to be followed carefully by OB so that she didn't bleed out. Mom and Dad and I sat down for a long, long talk about how on earth to get her teeth fixed and get her on some sort of contraceptive so this didn't happen again. I was gobsmacked by the difficulties they faced just getting her to the hospital in the first place.
I thank the gods who on my birth have smiled. If I lived like she does--and, not to put too fine a point on it, but it's in a house that isn't square and is about six hours away from the nearest clinic, where the weather reports are all in a language that was used as code in World War II--I would be dead by now.
She's lucky she's not. She's lucky one of her parents can read. She's lucky the U.S. Government, in its unending wisdom, decided that several million dollars spent fixing her various problems was a better investment than a couple hundred spent preventing them in the first place.
Fluoride, people. Regular dental visits. If you can afford 'em, do 'em. If you can't, well, you might be lucky enough to find a clinic or university nearby that can do basic care for cheap. If you find yourself flush, you might help a friend get a cleaning.