The reason I hesitate to count all beverages is as Jill mentioned: some beverages just don't hydrate you. In my freshman year of college, my mom gave me a case of individual bottles of orange juice (it was real orange juice, not orange-flavored stuff, but one of those kinds that doesn't need to be refrigerated until open - like
Welch's or something). I started carrying them around instead of water bottles and drinking them constantly, because I thought it would be healthier and give me more vitamins.
Then one day in class, I passed out - face first - onto the floor (from a standing position - ouch! Fortunately nothing but a few bruises). I went to the office to sit down and had more orange juice to get me going - figured it was low blood sugar from not eating a full breakfast or something. A few hours later, I passed out again, so I went to the doctor. He quickly figured out that I was severely dehydrated, and hooked me up to an IV with saline solution to get some fluid back in me. After about a gallon of the solution had gone in, he said I was free to use the bathroom, because he thought I was probably desperate and that my bladder about to burst. Not so - I told him I felt fine, and he was astonished. He said he had never seen someone so dehydrated!
So... that's the long story for why I am wary about counting things other than water. Juice can be very high in sugar, which means it's not as hydrating as water alone.