I think there is an important distinction between a MSQ+Instant level-up potion, and a potion that allows to level any class instantly on a same character. Both kind exists apparently on the Chinese server.

Even I would prefer another ingame answer to the issue, I can understand for a character to be allowed to use an instant-level up and MSQ skipping for a unique single class so he can play the new current content. But I'm certainly not in favor of potions that allows to level instantly any other classes after that on the same character.