As a Marriott multi-week owner, I seem to be in the minority here when I say that I find what Marriott did to be wholly unacceptable. Marriott has two separate roles at our resorts. The first is to clean our toilets (as PerryM used to say) and the second is as an owner of some of the units. The two are completely distinct and what I decide to do with my week, I expect them to honor it as part of the first role that I've hired them to perform. If I decide to deposit my week into into interval, I expect it to be used by Interval exchanger, not to be unilaterally taken by Marriott simply because they've sold an Encore week as part of their second role so that they can simply sell more points. This was not an emergency situation or anything like that, this is simply greed by Marriott.
Perhaps an analogy would help people to become more passionate about their rights regarding their owned week. Lets say I have a vacation home and I've hired a cleaner/handyman to maintain my vacation home. For one week, lets say I've swapped my vacation home with another family's home and I've told the cleaner/handyman to give them my keys for that week. The week before the house swap a really bad storm comes in and damages some of the homes in the town but my vacation home is fine. Afterwards, I've found out that the cleaner/handyman called up the other family a day before they arrived to say that my vacation home is not available due to the storm. Now, the cleaner/handyman is successful and so he has bought a number of other vacation properties across town over the years. And some of them have been damaged so he allowed his own guests to use my vacation home so that he didn't have to refund their money. He never told me he was doing this and I had to piece all this together by doing a bit of detective work.
If this happened in real life, I'm sure we would all fire that cleaner/handyman and hire another one called "Park Hyatt" or "Four Seasons" or "Conrad/Hilton". I, for one, would pay more to hire a cleaner/handyman that was trustworthy and wouldn't put their interests ahead of my own.
The bottom line is that I think we should all be outraged and have the board of the affected properties put Marriott in their place. I do not own at Hilton Head but if this happened at one of the resorts that I owned at, I would be much more vigilant and active about this situation.