because the only time you most probably wanted it is when you are away from the comfort of your dsl/wifi. And, most of that time, you are in a place without a gprs/3g connection and if there is, it is most probably crappy.
I am saying this because I am for SMS. Let us not forget that SMS is what makes the mobile networks such a big shot! So do they really care if you have crappy mobile internet connection? Maybe, but it’s not their priority. Just saying.
But on the other hand, mobile internet is quite a handy tool. I used it from time to time when I need to check email when I am out and away from any internet connection. I also used it when i get a picture of something interesting and i cannot wait to be home to upload it to the website. But i do not need unlimited surfing for these purposes.