My husband doesn't know that he has dementia and is never alone so this couldn't happen but I can see it being a problem for someone who does,they could use it as a excuse for unsocial behaviour,
My husband was fine with saying he had dementia but would not accept the word Alzheimer's. If he saw anything to do with Alzheimer's it always seemed to be in the late stages and he would say I don't have that do I? I would always reply by saying dementia comes in many different forms, that is just one of them. Not lying just telling the truth. He appeared to be very afraid of the word Alzheimer's so I was not going to add to his distress.
My husband was never on his own but I felt the card in his wallet a safety issue just in case and I also carried a card saying I was a carer with contact numbers on, in case something happened to me and someone else would know there would be problem.
Of course if my husband had not known he had dementia then his card would not have existed but I would have still carried one. I can't answer for others but my husband used his in the assessment centre to explain there was a reason for his behaviour not as an excuse to behave badly.
Caution is my middle name,
Jay