National Archive File Format Time Bomb
This example perfectly illustrates the problems with proprietary formats. Once the software that interprets a proprietary format vanishes any information written in it is gone. Okay, it's not gone gone. I am sure you can get a bunch of good cryptanalysts to pour over binary dumps of these files. Eventually they will crack it - if your information is worth the cost, that is.
This is why we need open standard formats such as ODF and reject pretenders such as OpenXML. Just because the name has "open" in it does not mean the information to completely read and write OpenXML is freely available to the public. This makes OpenXML a proprietary format dispite the name.
OpenXML should be placed where it belongs - the rubbish bin.