I was always told that when growing up but I never understood how drinking soda would actually cause a net loss of fluids. A quick search shows that my suspicions were correct. Even though drinking soda doesn't hydrate you as much as drinking water, it does not dehydrate you.
If you truly believe that these will net-dehydrate you, it should be pretty easy to prove - just survive for 1 week drinking only soda/juice/coffee. If you die of dehydration, we'll admit you were right :)
I've survived for years drinking mostly iced tea, with a soda in the morning instead of coffee. I can count the number of times I drink plain water in a week on one hand (after exercise, basically)
I can confidently admit that when I was a stupid teenager, I had consisted on Soda & Coffee and potentially only getting raw water content absorbed from foods for multiple weeks on end.
You would need to drink a massive amount for it to be lethal.
Instead what you see are obese individuals because drinking soda has the net effect of making you more thirsty, causing people to drink more soda. Which is exactly what the soda makers want you to do, and will fund studies to make you believe otherwise.
[1] http://www.bbc.com/future/story/20140402-are-coffee-and-tea-...