The helpers will handle the javascript validation for you. But you really need it in both places to have a high quality app.

>AFAICS, if you want server and client side validation with ASP.NET then you have to repeat yourself (once in C#, once in JavaScript) although for a straightforward MVC app the helpers will do it for you. ISTR that with node.js you can use the same javascript on the server side as well - but that's another story.