This ^
I know that both myself and my husband currently have photos attached to our
MDE accounts, and all active magic bands or AP cards. Whenever we tap in at an attraction, our photos pop up on the screen. So, while you might currently get away with using someone else's magic band & genie+, you also might not. If the person happens to have a photo attached for any reason, you will likely be turned away at the LL tap in point for an attraction (unless you can provide your own band/card showing that you actually do have an active LL window). But I could definitely see Disney eventually attaching photo identification to all bands and cards, in order to prevent this sort of swapping.