AND a room in a property that is otherwise sold out. Do you like sleeping on park benches?

As to the OP's question - I have used the guarantee several times. And not always had to pay the rack rate to do so, ie if I wanted 3 nights at a property and the middle night was sold out, I have been able to get that middle night at the average cost of night #1 & #3 - but this is an extra perk, dont assume you will get it.

I have also notice that sometimes a property will try to squirm out of honoring the guarantee. This isnt a recent marriott thing, it happened under SPG too.

Oh and last thing - just because a property is a resort of timeshare, dont assume they automatically WONT let you do an override booking. they might not - or then again, they might.