The difference here is that this Gengar will be legal with Pokemon X and Y game tournaments, which have a tendency to disallow anything transferred over. That's why they're doing it, and it's the same reason they gave that event Tyranitar Ice Punch. Won't affect simulator play though
But said tournaments are typically doubles, and unlike in singles where it's a coin toss decision between the two, Sludge Bomb is undoubtedly the superior choice in doubles.
Not always true. Spread attacks are a big part of Double Battles. Remember the Pachirisu using Champion's Gardevoir had Dazzling Gleam specifically because it's a spread move. It's spread moves that makes the move Wide Guard a popular move.
While that may have some truth in general, in Gengar's specific case Sludge Bomb is a lot better, since it's more handy for dealing specifically with Fairies. Poison moves are meant to deal with individual Pokémon, they're not something you just use whenever unless it's your only source of STAB.
Alright, already! I'm sorry I was unaware that Gengar had Sludge Wave before Gen 6. I missed the memo. Sorry about making you mad.
Not mad, just having a sense of "thought I say that already". Sorry for the confusion, that kind of thing isn't easy to translate to text.