It is one of the few perks of a incurable disease which requires painful medical treatments multiple times daily and can shorten your lifespan by 15 years, cause blindness, kidney failure, liver problems, extensive nerve damage, and other terminal health problems. The truth of the "diabetic card" may vary, as with other excuses.
Diabetes Card: "My blood sugar was 350 mg/dl last night, and I was unable to go to bed until well past 3 AM."
Professor/Teacher/Employer: "Ah... well, look out for yourself."
Teacher: Why havn't you done your homework?
Child: Aw sorry miss, im diabetic
Teacher: Ah okay
Teacher: Okay you failed your NAB for the 3rd time...
Child: Sorry miss, im diabetic
Teacher: Ah okay, i'll get you a re-re-re-resit