I think a lot of the back-outs -- by renters, by relatives, whoever -- occur because of cost. A lot of people focus on how much they save on one particular part of a vacation, and only later focus on the overall cost -- which is large.
At first, it sounds great! You're getting a free room, or a room at a big bargain -- and not just at a cheap motel, but at a deluxe on-site Disney resort! What could be better than that?

But then, they start looking at airfares...which have gone up due to fuel costs. So that's $300-400 apiece.
That's okay, we tell them, you'll make it up with the Dining Plan -- it's only $38 a day for more than you can possibly eat! And they think:
Who in their right mind would pay $38 a day to eat???
So we tell them, "No wait, you'll see!
DDP is a great deal -- if you don't get it, you'll probably pay $50."
"Huh...no, not for the family, $37.99 per
adult per day. But hey, the kids 9 and under are only $10.99! Huh? No, you have to get it for everybody in your group, for each day of your stay...but you'll save money!"
Then we start explaining APs and 10-day no-expire passes, and they start adding all that up, and asking questions like
"Whadda you mean $60 a day doesn't include water parks???" and
"Why do I need a Park Hopper again?"
Pretty soon, figuring airfare, DDP and passes, they realize they are looking at $150 a day
per person...
IF they have a free room.
Then, they remember your daughter's closet -- you know, the one with 19 Disney Princess outfits?

They remember they haven't included any trinket money, or any half-yards of beer, or Leave-a-Legacy, or...
A Disney vacation IS very expensive, and it's fast becoming out of the reach of many average families. That's too bad, but it's the reality. Sometimes when we're trying to do a "good deed," we're really inviting relatives on vacations they can't afford -- or don't want to spend that kind of money on. Maybe they CAN afford a $5,000 vacation, but they want to do it cruising or skiing, etc...not at Disney.
They can't afford it -- or don't think it's worth it (to them) -- and next thing you know, they have something terribly important that they need to do on those exact dates: "Sorry...I know what you said about cancellations...but we CAN'T go!"