The problem is that canceling is all or nothing. You can't change from a party of 6 to a party of 4, for example. Maybe someday but in my experience, that option didn't exist when I tried a couple of years ago. I could cancel the whole thing but I couldn't modify to reduce the number of guests.
And while I agree that members should not be allowing random strangers in on their tickets, I'm not sure how
DVC would police that. It would require changing the registration process to include all guests' names expected to enter under the member's tickets (and slowing down the registration queue) and then proof of ID for everyone would be necessary at the gate the night of, which would slow down entrance significantly.
In the long run, as
@Brian Noble pointed out, the problem ultimately lies in there being more members who wish to attend than there are available tickets. Not everyone can get AKV value studios. Not everyone can book BWV resort view studios. And not everyone who wants MM is going to get tickets, no matter how DVC decides to distribute the tickets.