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i personally don't think it's a great ability, but it can have it's uses, for instance, zoroark's typing gives it an immunity to psychic attacks and resists ghost and dark, so if the last non fainted pokemon in your party is something that is weak to a move that dark resists, or is immune to, like a psychic type attack, and is choiced, that essentially gives you AT LEAST a free switch, and a turn to boost or hit something on the following switch, maybe even another turn if your opponent is a bit confused another strategy i can see is a boosting set like this

zoroark@liechi/salac/petaya berry
adamant/modest/or any of the speed boosting natures
evs???
substitute
swords dance/nasty plot/agility
sucker punch/sucker punch/coverage move
pursuit/night slash/tauntnight burst
aerial ace/coverage move

Physical/Special or a mix of the two for a mixed attacker

the whole premise being with clean switch (or a switch into a psychic move) and illusion up you should be able to get at least one sub in, when your opponent switches to counter your "threat" and isn't aware of your pokemon being zoroark, you should be able to at least get one stat boosting move in, probably two, if the first switch is a choiced pokemon, (illusion might still hold with a sub up BTW), then just keep subbing until your preferred berry activates and proceed to use a swiffer and rip into your opponents team

illusion relies on prediction, and a team to support it, it'll have it's uses, its just not as straight forward as something as moxie

**sorry for the long windedness, hopped up on caffeine**

Hello =3

Okuu would never....ugh! D<
Hello new person! ^_^

They're given Fighting-type moves to balance them out, due to their weakness to Rock and inability to so much as dent Steel.

o-o 'Be kinda weird.

Staraptor can counter Grass, Bug and Fighting. Onix can cover Electric, Flying, Poison, Bug, Fire, and I'm sure there's more.

I know that but I'm talking about how they cover their weakness' with their moves.
Close combat takes out Ice Rock and Steel so Staraptor doesn't even have to worry about that...hell the only thing it has to really worry about is Electric.
whereas Onix has nothing to help it against a Ground or Water or Fighting type.