ASP Form Data Server Side Validation script uses one function to validate multiple form input data types.
There are a number of arguments for client side form validation:
1. Less network traffic making for a lighter load on the server.
2. Faster fulfillment of form requirements.
3. You can send users funny notices about their inability to fill in a form.
For all of it's advantages though client side validation is not as secure.
Of course for a real over-achiever you could use both client and server side validation, or even use AJAX for validation so you can have the security of server side validation and the immediate response of client side validation.