It was great back in the day when the Diamond desk was able to work with sold-out properties to secure an award stay. But with that perk long gone, the 48-hour reservation "guarantee" is rarely useful since it typically books into rack rate. There's almost always a better option (e.g., staying elsewhere) than paying full rack rate. Furthermore, I frequently find that the guarantee isn't available, even when there's no apparent "extraordinary demand" that would preclude it. As I recall, there has always been a published cap on overbookings. But when the option to invoke guarantee was made available online (as opposed to requesting via Diamond desk), perhaps more people took them up on it?