Why use a DFA instead of DIP1000?

[Walter Bright]

Requiring the language to remove all dead code before checking for errors means 
the entire program must be subjected to DFA. The result is compilation will slow 
down dramatically. It means separate compilation is no longer possible.

As a pragmatic choice, D is designed to not require DFA. It is a deliberate 
architectural choice, not a bug.

That said, an optional DFA can be quite useful in detecting otherwise hidden 
programming errors. But I'm not convinced it's a good idea to require it in 
order to successfully compile code, as your opening example suggests.