If a system left truly random numbers in uninitialised variables and applications started making use of that functionality (even if undocumented), then I think you'd need a very good reason to change that behaviour. But since I can't imagine any sane system ever doing that, no, I'm not proposing that we need a user contract on that point.
Contrast that to the ext3 behaviour. While, yes, the behaviour of fsync() on ext3 did result in people being even less likely to use it, the fact that ext3 also made it possible to overwrite a file without having to go through a cumbersome sequence of fsync()ing both the data and the directory made it attractive to application writers. That behaviour dates back far beyond Firefox 3, as demonstrated by people's long-term complaints about XFS leaving their files full of zeros after a crash. ext4 now provides the same ease of use because people made it clear that they weren't going to use it otherwise. Future Linux filesystems are effectively forced to provide the same semantics, which is a good thing.