I do not understand this limitation. If there were 3 comments on my answer, and I want to respond, why should I not be able to alert all 3 to my response, and put the whole response in one comment? It seems wasteful to just do 2 comments.
I think it's probably a feature that can be abused pretty easily. A desperate user might start mentioning the names of everyone he can find in an attempt to bring attention to the question.
there's no valid case for "I need to notify a large number of people" -- at least not one I will support by writing software for it. – Jeff Atwood♦
the comment box explicitly disallows submission of comments with two @name notifications now, FYI
So no cake for you