Just a comment on the notion of law vs. reality mentioned above.
I don't have any insider information from someone in law enforcement, but in my experience, you don't get pulled over or have to interact with the police unless you give them probable cause. If the law says they can't pull you over just for being registered with the MCPP, they're not going to do it. If you give them probable cause and DO get pulled over, maybe look up your info, see that you have a medical card, and use that information to help in the investigation. But if you don't give them probable cause, you're probably not going to get pulled over. That's the reality.
Yes, there are exceptions to every rule, and yes, there are bad cops who don't follow the rules. But the reality is, these exceptions are very rare.
Again, this is all based on my own experiences. But i'm old, and have lots of experience.
I understand and agree with the opposition to the principle that allows cops to see if you have an MCPP card. But the reality is that the vast majority cops have a lot of better things to do with their time than hassle otherwise law-abiding citizens by using a tiny loophole in the law that might allow them to do so, but won't hold up in court because the law specifically says that holding an MCPP card is not probable cause for anything.
Bottom line: if you don't want to be hassled by cops, don't give them probable cause.
If there is some data that shows that MCPP card holders are getting pulled over at a rate higher than non-card holders, I'll reevaluate my opinion. But until then, I believe this is a non-issue.
Disclaimer: I'm in Chicago, where the majority of Illinois' population and cops are located. If you're in a more rural, less populated area, and your experience is different, you have my sympathy. But that experience is not typical - it's an aberration.