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I guess it could be, but only because no-one's expecting you to use it over a regular Prankster Sableye. In my experience, a lot of people try to shut regular Sableye down with a status effect of some kind before going for raw damage, even if it's one of your Will-o-Wisps Magic Bounced back at you.
I think the last thing someone would expect when they whipped out their Espeon would be a Shadow Ball to the face from Sableye