Popularity has nothing to do with it anyway.
Now earlier Kid said, if its sheik, use aforementioned strategies for sheik, when she switches to zelda, insert strategies regarding Zelda.
This is true but the simple fact that its being performed in the middle of a match alone is enough reason to refer to the characters as Sheik/Zelda.
Sheik is an integral part of Zelda, it isnt a case like ZSS where the number of changes is limited.
Its the same as PT, except you arent forced to switch at a certain time limit.
Otherwise, we should refer to PT as three seperate pokemon, because honestly, its not often that a forced switch occurs.