So... By that logic, sleeping must also be bad for ones teeth. Unless of course they talk in their sleep.
It would explain the morning breath phenomenon.
Who is most likely to need-a-drink? Some one waiting in line alone? Or someone giving a speech??
Don't confuse dry mouth with dry throat. One doesn't necessarily have anything to do with the other.
I don't think this dentist's advice could hurt you if he's wrong and could only help if he is right. I do know the habit a lot of people have of guzzling a lot of sugar loaded soft drinks, tea, coffee, etc while working can't be good for your teeth, so just the act of replacing that with water could do a lot of people's teeth a lot of good. And don't forget that even if you use a sugar-free sweetener in your tea or coffee, if you are adding milk, it still has sugar in it.