Metformin does decrease the incidence of developing diabetes, but there are inherent risks in taking the medication--I do not know Allen's creatinine, or his liver status. Both of those could be contraindications to taking metformin. In addition there is the cost associated with metformin. At this time, I do not recommend my glucose-intolerant patients who have heart disease to take an additional pill--they are already usually on an aspirin (and sometimes plavix <clopidogrel>), an ace-i or ARB, a beta-blocker, and a statin. And that is if they do not have hypertension--if they have hypertension, they are usually on even more medications--so I usually do not recommend my patients taking another pill on top of all the others, if they are "just" glucose-intolerant.
That being said, I am NOT Allen's physician, and I will defer to his physicians completely, as they are taking care of him--and know his situation better than I do. In addition, they may have knowledge of trials that have not been published that they are involved in that may show benefits of a medication. So, again, I am not trying to argue with Allen's physicians--just trying to show a point of view.