Wait. I just figured it out. The reason kids use text messaging instead of phone calls is because you can do a text message under your desk, in the back seat of the car, at the restaurant, and 90% of the time no one is going to call you on it, other than to say "what are you doing?". But if you were sitting on the phone talking for the same amount of time, well that would most definitely get you in serious trouble (1000 text messages / month, huh?), especially in school where you really can't just sit in class jabbering away on the phone. I notice it's also a nice handy way for kids to semi-diss their parents, with help from the Youth-Are-Gods media culture ("gawd, dad, you just don't GET it, do you?").
If I were a parent I think I would eliminate texting from my kid's toolset, at least until they are out of school. Then, feel free.