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Well, most of them will contain some fermentable carbohydrates, so not ideal. However most people don't consume beer that frequently so it's not that important. If you were drinking so much beer that it was affecting your teeth you probably have other more pressing issues...
Define a crazy amount of beer? The thing with tooth decay is that it's not so much the amount of carbs that are the problem, it's the frequency. Sitting supping one pint of beer very slowly over a 6 hr period (I mean it's not likely but still...) would do more harm (to your teeth) than guzzling 5 or 6 pints in a few minutes.
Not many alcoholics drink beer, not enough alcohol and too much "bulk", they usually go more for wines or spirits.
Generally it's because they don't brush their teeth at all. They regularly throw up, so a nice regular acid wash over the teeth, their diets are normally pretty poor with a high carbohydrate intake.
Will it make them decay? Probably increases your decay rate. Depending on the rest of your diet it won't make a huge difference.
Will it make them fall out. Not really, that'll need you to have periodontal disease which can take decades to get to the "teeth falling out" stages.
Feels like we're going in circles here. It's not the amount, it's the frequency. Cola has a lot of acid (and sugar if it's not diet) in it Sipping a can over a few hours is really really bad. Guzzling a can down in a oner once a day is probably not as bad as sipping beer over a 5-6 hr period.