That only holds for numbers already part of the dump. If you submit an unknown phone number that is not contained in the leak, hashing it before sending it will increase data privacy.
While the idea is nice, I'd rather not use a bookmarklet that tracks me through a unique ID across websites and not only keeps a list of pages that I used it on, but also of pages that I came from: https://github.com/cameron/squirt/blob/gh-pages/bm/squirt.js...
In fact, it tracks whenever you use it and sends the URL along with all GET parameters of the page that you are browsing, including the referrer(!), your user agent and your IP to Keen.io: https://github.com/cameron/squirt/blob/gh-pages/bm/squirt.js...