Because I'm generally writing LINQ. You can use LINQPad to write ad-hoc queries and verify they do what you think without having to do the compile/load/test dance.

You can use VB.NET or C# to write procedural code and create user-defined types and aggregates in SQL Server.

Using JSINQ, however, you can write similar SQL queries while programming in JavaScript. This means you now have a query language in JavaScript that you can use against arrays and DOM node lists.

An alternative - again, if supported - would be to create a view that works the same as the derived table, and use this in your query instead.