Ok, but what is the down-side to throwing an error for an invalid macro name? I can't see one.

The script is going to fail either way, but throwing an error is far easier to diagnose than a script which continues to run but does something odd.

Considering the majority of macros-in-strings scripters are likely to be newbies and Shawn I think that something that's in your face saying "you've typed something wrong here" is much more preferable.