I'm not making an argument, just an observation.
I do agree that you can't really play a playable Void Elf as a High Elf, though - they don't really share in the essential divide between the High Elven exiles and the Blood Elves, they have their own cultural (and physiological) divide between themselves and their kin that's part and parcel of their story.
Alleria's story and role in the Void Elves' story is an exception to the basic rule, and doesn't matter overly because Alleria is obviously an NPC and non-playable. Any playable Void Elf has a different story. Though I guess you could RP yourself as an exception to the rule, as well; I'm not personally opposed to that although I think it falls into the far-end of acceptable RP/OC behavior, though I'm not big on WoW RP myself so I don't know firmly what is or isn't acceptable.