The ability descriptions, the apparent lack of animations (based on the pictures) and the in-game text all sound more like Air Lock's mechanics (not in the sense of cancelling out weather, but in that they affect all Pokemon currently on the field so long as the Pokemon with the ability remains on the field). I also wouldn't be so sure they're mutually exclusive (although if they were weathers they surely would be). That being said, assuming Xerneas/Yveltal aren't walls, both Giratina formes will definitely be threatened and may see less use. Actually, with boosted Dark type STAB, Flying STAB and hopefully a good Fighting coverage move (not Focus Miss please) I could see Yveltal becoming the premier wallbreaker of Ubers, especially if it can get past Chansey. I could also see it being rain offense's new best friend (Dark/Flying STAB wrecks Latias, Latios, Grassceus) and if it gets Hurricane (that sounds terrifying) Kyogre would be its best friend too. Of course, for it to beat Latias and Latios (Thunder would wreck it back) it would need to be the fastest offensive cover Pokemon to date by a significant margin, which seems somewhat unlikely (it could run a Scarf, but where Scarfs go Genesect already OHKOs the Lati twins and hits Grassceus hard, while being the awesome Scarf user it always was). Its typing also suggests the best Ghostceus check to date, which may give rise to an increase in the popularity of other Arceus formes. I could also see it being the bane of spinblocking (hi S-rank Ho-Oh), unless a new Ubers-worthy Ghost that can handle it is introduced in Gen 6 (I'm personally hoping for a Steel/Ghost type with levitate; 4 immunities, 7 resistances, NO effective weakness in rain and laughable entry hazard damage). With Xerneas it's a lot harder to speculate what its main niche will be, especially as so little is known about the Fairy type. Boosted Fairy type STAB will undoubtedly hit Dragon types like a truck, but then most of the tier's numerous Dragon types can already boast as much. Of course, a powerful Fairy type signature move will have definite advantages; hardly any Dragon type moves OHKO Dialga, and a OHKO on Giratina requires Outrage or Draco Meteor, both of which have major disadvantages. Still, having the single best way of dealing damage to dragons hardly seems enough to carve out a significant niche, so it remains to be seen what resistances Fairy has, what resists it, and how well Xerneas' stats and movepool complement this typing.