The Rock-type would neutralize Fennekin's effectiveness against Chespin. The point of a secondary cycle is to make them strong against each other without interfering with the already established hierarchy of Fire-Grass-Water.
Remember, we did have that messed up Pseudo-Triangle in Gen 4. So I wouldn't doubt if Game Freak did that again.
Yeah, that one wasn't entirely thought out, but it did give us some really good starters at least.
Playing: Fire Emblem Awakening, Pokemon White 2, Bioshock Infinite I believe in everlasting life in Heaven.