Why your language needs macros

A couple of days ago someone posted yet another "why do I need macros?" thread on Hacker News. The usual arguments and unconvincing examples arguing for macros were posted. Noted absence were the arguments against macros (all of which seem to be of the variety "programmers are too dumb to understand someone else's macros" - if you were at the Great Macro Debate at ILC 09 you would have heard it stated more eloquently).

I've been thinking about macros in terms of domain-driven design lately, kicking around my idea of "domain onions" (I'll write more about this later), so I decided to post my current thoughts about why every programming language that aspires to be general-purpose needs macros.