I love...love having them. It is our "government photo ID" card of choice. Book passport stays in the safe, unless the cruise line takes them from us on embarkation. Has happened twice now in the last year.
Many times I will tour in port with just the sea pass card, RCI credit card, some folding bills, and the passport card...all secured in my button down front pocket. I find it's all I need.
I have a tiny cross body purse just big enough to fit my phone, and small items when I want to take them, but the cards and cash are always separate and in my pocket.
This works for us, but we are not all alike. 😉