I'm not quite sure how being able to extend syntax is a theoretical problem. It was just a basic example for brevity.
There are many more examples of macros that solve real world problems, such as writing a compiler for an embedded DSL that is tuned to solving your real problem that allows expressivity and brevity that you would never see without the use of macros.
There are many more examples of macros that solve real world problems, such as writing a compiler for an embedded DSL that is tuned to solving your real problem that allows expressivity and brevity that you would never see without the use of macros.