This is very interesting, and something I hadn't thought about.
I have an unused ticket which I just linked up in
MDE, and I checked to see what FP+ are available at MK tomorrow. I could get both Meet Anna & Elsa and the 7 Dwarfs Mine Train tomorrow evening.
But if I were at WDW right now (and had no extra unused ticket like I do here now) and if I had FP+ reserved tomorrow for a different park, I would have no way to find out that MK has these hard-to-get FP+ available?? (If I could find this out, I might welll want to switch my park day, especially if we had no ADRs scheduled.)
Doesn't that seem like a major flaw?
But maybe not from Disney's standpoint, though, since a major goal of FP+ is apparently to "lock guests in" to a particular park. Hmmm...