Depends on what you are pulling...I think this stems from the complexities coming out of genders.
So if bisexual is you fancy a bit of males and females, then what about a man who identifies as a woman or vice versa... you might not fancy that.
Pansexual incompasses this and prevents confusion and awkward conversations I guess...
Couldn't care less whatever people want to call themselves. Just curious as to peoples reactions are as its more likely now that you could "pull" someone in a club without knowing their true/original gender!