When is it OK to drink them? Easy to remember answer: Never.
The problem is not that it's "never" okay to have a sugary drink, the problem is that it's a part of many people's everyday diet. A sugary drink should be a treat - something you have every once in a while not for legitimate sustenance, but to taste something (which to some people may be) pleasant.
Life is short, and none of us are getting out alive. A root beer every now and then may be better for the soul than it is bad for the kidneys.