The idea of yomtov shaini b’chutz la’aretz is something that I could never understand (not because it wasn’t explained) 🙂
I mean, there is no safek yom anymore, this we know. And there are more serious implications than simply those of convenience (or inconvenience). For example, the torah specifically says that succos is an 8 day holiday, so next friday A. why aren’t you oiver on baal toisif? and B. Shouldn’t you be putting on tfillin? C. Shouldn’t you be doing hakafos (the real ones) on Thursday