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Simply put: parenthesis are used whenever we suspect that there may be operator precedence issues. 

- Either because the user passed an expression containing several operands and operators to the macro. 

  To deal with this we need to surround the use of each macro parameter with parenthesis. 
- Or because the macro is used in an expression together with other operators. 

  To deal with this, the function-like macro needs to be wrapped in an outer parenthesis.

And yes there exists plenty of situations where we don't really need to worry about what the user passed, since it wouldn't make a difference for the macro. Or if they pass something weird and therefore get a compiler error, then that's not necessarily a bad thing.

But it is good to keep ones coding style consistent and analyzable by tools. If we always follow both of the above mentioned best practices with parenthesis, then we can also verify, by means of a static analyzer tool, that no bugs caused by missing parenthesis exist.

It's kind of the same thing as writing `{}` after each `if`. While writing the code we can conclude that the braces after `if` aren't needed in some cases, yet we know that if we go lax with our coding style and start skipping them, that lax coding style will eventually lead to bugs.