There actually is NO distinct cutoff between normal, prediabetes and frank T2. The line is wherever the medical establishment decides to put it. A couple of decades ago, the diagnostic cutoff was 140, because a given percent of the population fell below that number (don't remember what the percent was).
Then some studies emerged, primarily on the Pima Indians of Arizona, but also on other populations, which showed that the incidence of diabetic complications, primarily retinopathy rose sharply at FBG somewhere around 120 - 130, so the powers that be decided to lower the diagnostic threshhold for diabetes to 126. They were cautious about not making it too low because they didn't want to treat people who really didn't need it.
Now, they're looking at A1c as a gold standard for diagnosis, based on studies of complications as well. Heart disease is a major issue for T2s and some of the decision will be based on CVD statistics. The number that they're going for is 6.5, which to me seems pretty high, all things considered, but they're not taking individual variation into account -- just looking at overall populations.
The concept of pre-diabetes is a fairly new one, and establishing the low end of the spectrum at 100 is pretty arbitrary, but at least people who have good doctors will know they're at risk, and have the chance to do something. Avoiding an actual diagnosis through good control habits is essentially the same thing as saying you've prevented or delayed it, and can have benefits, such as when you apply for insurance.
In my own case, if current guidelines had been applied, I would have been diagnosed 2 years earlier, sparing me a lot of agonizing over whether I did or didn't have diabetes. (The previous year, I definitely didn't) But in the end, the best advice was given me by my dearest friend: don't worry about whether you have diabetes or not -- live as if you did, because that means striving for a healthy diet and sufficient exercise and healthy living -- it can't hurt you and it CAN help!