If it doesn't make sense for Xerneas to get a new type, then it doesn't make sense for Mewtwo to get a new form. Yveltal fans are still going to get Y regardless of the legendaries typing. Xerneas fans are the same, if anything, it will will just make the sales of X and Y more even probably. Also, if they went with Fairy/Grass and Dark/Flying for the types this gen, the legendaries would both have a type advantage in some way over each other, and if the starter evolutions types are correct, GF may very well be going with more balance in Pokemon this game. For every Xerneas fan I've come across there are probably like, 3-4 Yveltal fans... Dark as legendary typing has only been done with Darkrai and we haven't had this type on a mascot before either. So I honestly don't see why it's a problem if Xerneas had the new typing...