They're occaisionally helpful, particularly if the patient is non-communicative. What's REALLY helpful, is to have a list of your current medications (with dosages) with you AT ALL TIMES. Unless you have other medical problems,(espcially diabetes, as mentioned above) if you "go down", you'll be treated as a cardiac arrest, so in your specific case, it's probably not as useful as it is for the diabetics, or patients with certain allergies. Things docs really want to know are (besides the presenting history):
1) When you had your M.I., what areas of the heart were involved? What is your current E.F. (ejection fraction) on meds?
2) Do you have any drug allergies?
3) What are your meds and doeses? Don't forget to tell/write about aspirin! Blood type isn't that important, but might help speed things up a very little bit, since each unit is typed and cross-matched anyway.
4) Other medical problems? Past surgeries? Implants? Such as stents, pacemakers, aneurysm clips, hearing aid, cochlear implant.
Print this info. out on a small sheet of paper, using a word processor, save the file and update it everytime something changes. Try and get the EXACT spelling on your meds (some of them sound far TOO MUCH like each other to most patients. Laminate the little cheat sheet and keep it in your wallet, or a spy capsule, or if you use nitro, in one of those little cases.
Trust me, keeping this information with you at all times will make your care ALOT easier for anyone involved, and may save your life. Don't trust your memory to this information.