Like platinum suite upgrades, some properties are really good about Your24, and others are really bad about it and will hardly ever (if at all) approve it. Some of the properties that are good about it, and I've had a pretty good number (although I've also had my share of denials, particularly in places like NYC and resort hotels), will even combine it with a suite upgrade (which is not an SPG requirement). I doubt that Marriott will keep it, just like I doubt they will honor guaranteed 4 PM late checkout, although I think we have a pretty good shot at keeping most of the rest of the platinum benefits.
It's nice when it works, because it can save you the cost of an additional night, especially when you have a later flight.