Without the ability to future-proof being inherent in the format (like XML, which is self-describing), the sad reality of development practices in programming shops mean that one day, someone will make an undocumented change or take a shortcut that will tightly couple the binary format to the specific version of the code used to produce & read it. Which is fine, as long as you know that that coupling will happen when you're planing things. Not so fun when you have to go back and read a 3-year old file, only to discover that you can't.

Something that comes to mind is the old COM formats that MS-Office used to use. Eventually they had to abandon it (and not just because of the EU lawsuit) because it was unmaintainable, and no one understood how they worked well enough to not-break backwards compatibility for the next release.