Client side validation is just there to provide a nicer experience for humans. If there are conditions that you need to enforce for the sake of your server-side code or database, then you must do that in your server-side code.
Nothing stops a hacker from doing a raw submit to your server-side page and bypassing all the Javascript checks, including those done by jQuery and the validate plugin. The checks that you need to do on the server side include checks for empty required fields, cross-site scripting (XSS) and SQL injection. Google can tell you more about how to avoid those exploits.
