I think one of the reasons for the third argument (the array itself) is to permit chaining. Consider this:

    var publishedPosts = posts.filter(function(post){
        return post.published
    })

    var intervals = publishedPosts.map(function(post, i){
        var next = publishedPosts[i+1]
        return post.date - (next && next.date || 0)
    })

With the array reference you don't need an intermediate var:

    var intervals = posts.filter(function(post){
        return post.published
    }).map(function(post, i, arr){
        var next = arr[i+1]
        return post.date - (next && next.date || 0)
    })