My workplace are really supportive - we can apply for paid time for medical appointments at managers discretion - my manager and I have a great relationship, and I regularly do them favours with shift changes/extra duties etc so any time off I need medically is approved. This doesn't come out of annual leave/TOIL etc and whilst we do have company guidelines about how long appointments should be/when annual leave/TOIL should be used instead etc, I've never had a manager who made me take a holiday or work back time needed for any medical appointment, diabetic or otherwise. I do work for a very large company though so that may be why we have such 'luxuries' with it?
Way my manager puts it makes sense to me, whether I've been good/bad with my diabetes now/in the past is irrelevant to them unless it affects my ability to work, so why would they make it a challenge for me to attend appointments to help improve my control/deal with complications when ultimately attending the appointments will mean my ability to work isn't affected? =)