For me i think dual type moves should be rare but for me that's because they can give certain pokemon something interesting. I think more will come eventually particularly if one type combination becomes too strong. I could see a rock/ice type eventually to deal with Mega Rayquaza. (not given to many though as it would destroy too many flying types.)
But Mega Rayquaza has Delta Stream.. So the Rock-type damage would be weakened by that ability. Better make it Fairy/Dragon.. Because that would still hit it for x4 damage I guess.
What if they're trying to give almost every pokemon a unique Ability. That way, each pokemon is unique. It seems more likely. The "Pokemon with past Abilities" to "Pokemon with new Abilities) ratio is kind of being proportioned. A lot of them seem like signature Abilities as well.
This was happening a lot in GenVI. Not necessarily unique, but at least new.. I mean in GenVI, the regional dex got these new abilities: Bulletproof, Protean, Magician, Cheek Pouch, Gale Wings, Symbiosis, Grass Pelt, Fur Coat, Stance Change, Aroma and Sweet Veil, Contrary (not new, but the first with it being legal), Tough Claws, Mega Launcher, Strong Jaw, Refrigerate, Pixilate, Gooey
These don't lines didn't have a new ability: Vivillon, Pyroar, Pangoro, Meowstic, Dragalge, Heliolisk, Hawlucha, Carbink, Trevenant, Gourgheist, Avalugg, Noivern, Diancie.
Now take a look at the majority of those:
- Pyroar: Normal/Fire - New Type-combo.
- Pangoro: Fighting/Dark - New Evolution-method.
- Meowstic: Psychic - First pokémon with a branched evolution based on gender - where both gender have a different level-up set and abilities.
- Dragalge: Dragon/Poison - New Type-combo.
- Heliolisk: Normal/Electric - New Type-combo.
- Hawlucha: Fighting/Flying - New Type-combo.
- Carbink/Diancie: Rock/Fairy - New Type-combo. Diancie is the only GenVI with a Mega Evolution.
- Trevenant/Gourgheist: Ghost/Grass - New Type-combo.
So almost every pokémon in GenVI, with the exceptions of
Vivillon, Avalugg and Noivern (though it is the first
Flying/Dragon..) have something new in either ability or type.
It seems like they're really going for: "If we give each pokémon something impressive like a new ability or new type-combo, instead of yet just another Normal/Flying-type with Keen Eye, they won't hate us for making less each time.