If the dentist is being scientific, it's up to him to back up his claims about sugar and acid being the most (or only) important indicators for what's bad for teeth.
If you do some googling, it's commonly accepted that both sugar and acid are bad for your teeth, with plenty of justification. Some of the reasons are even covered in the article.
You're correct in that their classification of "worst" only applies if sugar and acidity are the most important factors. But they're clearly a large factor in tooth decay, so measuring the contents of various liquids and comparing them to each other is instructive.
> But they're clearly a large factor in tooth decay
Maybe, maybe not. The in-vivo environment might have many significant differences than some in-vitro setup. What's the role of saliva? of bacteria? of genetics?
Don't underestimate the power of the telephone game (http://en.wikipedia.org/wiki/Chinese_whispers). You need to go back to primary sources. It's very common for papers in software engineering, for instance, to confuse qualitative statements from earlier papers with empirical results.
I was just suggesting that the dentist might be familiar with the best science we have directly or indirectly, not even that he was acting scientifically per se.
