> I fall heavily into the use less powerful languages "liberties constrain, constraints liberate" camp.

For what it's worth, so do I. That's why my build system will allow users to set those constraints. It's a great idea.

> I'd be very interested in a real world example of this, though I know that class of examples is hard to come by.

There are many, but I'll say one: anything that requires iteration until a fixed point is reached. A concrete example of this is building documentation in LaTeX.

Or it may even be simpler than that. Maybe your build requires some sort of special code should two arguments have specific values. In other words, edge cases often require Turing-compete power to resolve.