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#wordpress believes there’s room for everyone: current_user_can( ‘exist’ ) always returns true
— scribu (@scribu) July 15, 2012
In fact, if you look at capabilities.php you’ll see that WordPress adds the “exist” capability after applying the filters to the original list of capabilities for the user, so it’s difficult to even hi-jack. Some might think the capability is useless but I think otherwise.
Many plugins nowadays, give you filterable capabilities for certain actions, like Regenerate Thumbnails. If I’d like everyone to be able to regenerate thumbnails on my blog, I’d filter that capability to exist. The “read” capability is very similar, but users are not allowed to “read” on private blogs, while they’re still allowed to “exist” :)