Actually, soda does contribute to hydration. At least according to a study that was done a couple of years ago. They found that almost anything you drink counts toward your daily fluid intake.
It was a pretty well-publicized study. I'm surprised you didn't see it. Here's a link:
http://www.ncbi.nlm.nih.gov/entrez/q...&dopt=Abstract
That's not to say that diet sodas are as good as water, just that diet sodas aren't as bad as people think.