Interesting topic, but i go with Old Jack, if nothing else, in being equitable. Now if your spouse is MIA how can you even know of you can itemize or must take standard deduction? I think constructive abandonment comes to play here. In one of the cases above the person didn't even know if their spouse was still alive, hence MFS might be wrong too.
I'd go substance over form and on the rare chance of it happening, i'd deal with it on audit. On the other hand, if the irs says the spouse filed as mfs and thus you must too, congratulations, you can now find the person for child support and/or alimony.