There are two forms of using macros in spec files - %{macro} and %macro. RPM supports second form since 4.0, IIRC.
In one of my packaged I used %_bindir macro and was told by reviewer to change it to %{_bindir}. OTOH, %configure is also a macro, but it is widely used without curly brackets.
What's the point of using %{macro} form for some, but not all macros?