As far as any and all arguments go to the usefulness or otherwise of Fairy type, Gamefreak are introducing a new type. There will be an impact, positive or negative, on the metagame, and that is guaranteed. What we can't say for sure is the extent of said impact, which is what renders most discussion entirely moot. We don't even know everything about the type match ups of this current type, and currently only know ~5 or 6 Pokemon who actually even possess it. 4 Pokemon which we know the entire statistic information for (of which the only two of genuine note would be Azumarill and Gardevoir [and even then arguably only in lower tiers]), and the others we know barely anything about. There are so many factors which have to be considered with regards to the usefulness of Fairy typing in OU, but as it stands the only thing we really know about it (I'm excluding the PokeBeach rumours), is that it's super effective against Dragons.