I had a fresh install of WP and the API to test out the backbone helper script.

Making a call to the backbone script in wp-api results in a broken url for collections and I presume models also.

posts = new wp.api.collections.Posts();

console.log( posts.url );

results in seeing ".../wp-json//posts" i.e. the wp/v2 is missing.

I have tracked this down to extras.php line 34 where the wp_localize_script sets up the WP_API_Settings and hacked it for my personal development