Yeah, I was sort of overreacting to the fact that an exception was thrown when an apparently perfectly valid (but hand-crafted) word document failed to load in the original version. 
However, my rationale is that there are instructions other than `MERGFIELD` in a word document, e.g. `CREATEDATE`. I'm not sure if these instructions will appear in a `w:fldSimple` tag, so I decided it would be more prudent not to eliminate such a possibility. 

Instead of producing a warning on any instruction not equal to `MERGEFIELD`, `_extract_mailmerge_instr` would produce a warning for any `MERGEFIELD` instruction that does not match the regular expression.