I guess I should know this, but help me out here:
Doesn't "either" essentially mean, "one of p and q is true, but not both"? If not, what is the purpose of including the word "either" in this expression? I follow the logic for "p or q," since "or" just means that one or the other (or both) could be true. But your comments suggest to me that "either" has no meaning, which should not be the case.