For a practical example of where this HTML encoding escape mechanism is useful, consider scenarios where you use HTML helper methods with ASP.NET MVC.  These helper methods typically return HTML.  For example: the Html.TextBox() helper method returns markup like <input type=”text”/>.  With ASP.NET MVC 2 these helper methods now by default return HtmlString types – which indicates that the returned string content is safe for rendering and should not be encoded by <%: %> nuggets. 

This allows you to use these methods within both <%= %> code nugget blocks:

As well as within <%: %> code nugget blocks:

In both cases above the HTML content returned from the helper method will be rendered to the client as HTML – and the <%: %> code nugget will avoid double-encoding it.

This enables you to default to always using <%: %> code nuggets instead of <%= %> code blocks within your applications.  If you want to be really hardcore you can even create a build rule that searches your application looking for <%= %> usages and flags any cases it finds as an error to enforce that HTML encoding always takes place.