It's not only the caffeine, the sugar substitute that they use has been shown in studies to be very unhealthy for people if they consume too much of it.
I don't drink soda at all, personally, and when I do it's always caffeine-free.
And btw, caffeine IS a drug, it causes a psychological addiction. It actually is quite highly addictive, it just isn't as bad (persay) as a nicotine addiction (as an example) -- the withdrawals aren't as bad and don't last as long.