HYPOTHETICALLY AS AN EXAMPLE:
I rent a two bedroom villa. I go on a trip with my brother & sister-in-law, as well as my cousin and her husband. We set up the MagicBands so everyone's fees are going to the same card because we will charging a lot of joint meals and feel it's easier to divvy out the costs at the end from one statement. DH and I come down with a stomach bug, and the other four decide to go to Downtown Disney for the day. "Here, I say, since we're just going to be lying here in bed, take my magic band, it has my DVC member card linked to it."
And this is one reason why they don't do that.
"In fact, I have three magic bands because we did a split stay this trip. Take one for each of you, then you won't have to worry about staying together to get the discount while you're shopping."
That's another reason why they don't do that. Note that that one works even if I'm not in bed with the stomach flu.
Yes, you say, but then they could just ask me to show a photo ID and that problem would be solved. Well, if you're carrying a photo ID, then whatever you're carrying it in likely has space for your card as well.