In my opinion, it is mandatory to validate data using client side and server side validation as a fail over process. The client side validation allows users to correct any error before they are sent to the web server for processing, and this allows for an immediate response back to the user regarding data that is not correct or in the proper format that is desired. In addition, this prevents unnecessary interaction between the user and the web server and will free up the server over time compared to doing only server side validation. Server validation is the last line of defense when it comes to validation because you can check to ensure the user’s data is correct before it is used in a business process or stored to a database.

Honestly, I cannot foresee a scenario where I would only want to use one form of validation over another especially with the current cost of creating and maintaining data. In my opinion, the redundant validation is well worth the overhead.