I am not really being disagreeable. I do definitely agree that it is useful in some very niche situations, like if you are going to a park later on your arrival day or if you have to miss the morning due to a recital or competition as you mention and you aren't planning to park hop. But even on these "successful" stacking days someone would have been more successful and gotten on more rides, both standby and LL by arriving to the park earlier and using it as designed. I am just responding to everyone here and elsewhere saying that stacking is back and it's this amazing new trick when it isn't that amazing (and depends heavily on Disney not changing anything.)
I remember back when the
lightning lane scans turned blue 15 minutes after your time ended, not 2 hours. They could go back to this at any point, especially if too many people are doing it. It breaks the system if too many people are booking lightning lanes but then not using the times until hours later. Guests should treat the time as a safety valve in case they are delayed or late, but not rely on it for a successful day or to try and me more efficient than the normal usage (and anyway it obviously won't be more efficient than a guest who can get there in the morning)
I believe I have seen it tested in a few videos, and unless something has changed very recently, after an expiration of a tier 1 LL on a day when you have not entered a park or redeemed a LL, you can only select rides at that same park.
Ex. You can use a wrench as a hammer, though it won't work as well as a true hammer would. But I wouldn't go out and buy a wrench just to use it as a hammer. It simply isn't as efficient in most normal situations.
ETA: One big thing that bugs me in normal life is obvious inefficiency. I can spot it pretty easily and it bothers me, which is why I made the responses.