The ribbon is merely a way to quickly administer your site. It's actually there for self-hosted WordPress.org setups, too (though you can turn it off with a filter).
But think for a few minutes about why you want it hidden.
- Do you want it hidden from just you because you don't need it?
- Do you want it hidden from you readers so they aren't reminded you're hosted by WordPress.com?
- Does it not fit with the aesthetic of your site?
If you want to hide it from your readers, keep in mind that "The only people who can see it are those who also have WordPress.com blogs and are logged into WordPress.com when they visit your blog."
If you want it hidden from just you or if it doesn't really fit with your site, you would be best suited migrating to a self-hosted installation. You can get on a shared host for about $6/month and can get a custom domain for about $8/year depending on the host. Migrating is pretty easy, and then you'd have full control over the presentation of your site without needing to buy other premium add-ons.