Concerned Doctors want us to stay well hydrated. Staying well hydrated doesn't improve salivary gland function, it only prevents dryness from getting exacerbated. I drink an average of 4 cups of coffee most mornings and plenty of fluids all day long, so I'm not at risk of becoming dehydrated. Coffee helps ease my lung symptoms (bronchodilator) and my brain fog. It's rich in anti-oxidants and reduces the risk of various cancers. It's one of the few things that actually make me feel and function better.
Here's my rant, lol. I don't trust health advice from experts who demonize foods. Remember when experts sounded the alarm that eggs, salt, fats, and butter were the great dietary evils? Now eggs, salt, and fat are not only ok, but necessary/beneficial. First butter was bad, margerine was good, Now margerine is bad, butter is good. Chocolate causes acne. Remember that one? Now studies show chocolate has no causal effect on acne. Unless I have a specific food allergy/sensitivity, I'm not going to put myself at risk for nutritional deficiencies by restricting my diet. I eat a variety of foods and beverages, everything in moderation.