There is one killer-reason to validate on both the client-side and on the database-side, and that is security. Especially when you start using AJAX-stuff, hackable URLs and other things that make your site (in this case) more friendly to users and hackers.
Validate on the client to provide a smooth experience to early tell the user to correct their input. Also validate in database, (or in business logic, if this is considered a totally secure gateway to the database) for security for you database.
