Tora Ziyal wrote:
Except they do have a last name. They just don't use it much. (See quote and link in message #106.)
While that's true (and I think Mountbatten-Windsor is a super cool name), none of the royals seem to use that in public usage. As mentioned, Harry and William used Wales, and their cousins Beatrice and Eugenie use "York" as a last name at school. I think Charles and Andrew used Windsor when they were in school, or at least I remember reading that somewhere.
"Mountbatten-Windsor" is a really weird concept. It's not really their legal name (like on passports and stuff), but they can use it if they need to. I don't pretend to know how they use it or would use it in day to day life, but as far as their legal name is concerned, that's not part of it.
It must be their legal name. The website said one occasion for its use is "upon marriage". The first time the name appeared on an official document was the marriage register when Princess Anne got married in 1973. Can't get more legal than that.
And, yes, Mountbatten-Windsor is very cool.