A coffee or two in the AM to get stuff going? Not the end of the world. Same with soda. But people who suck down coffee/soda all day, that starts to be a problem. Yes, the liquid in the coffee hydrates, but at
@LAKid53 said, the caffeine sucks water back out. Alot of people aren't properly hyrdrated to begin with, and it gets even harder when on vacation. And when you are running, if you feel thirsty you are already usually under-hydrated.
Which is an interesting situation when you look at energy gels and such that are caffeinated for use during long runs.