alt Hacker NewsWe build a code review product that interfaces with over a dozen SCMs. In about 20 years of writing diff parsers, we've encountered all kinds of problems and limitations in SCM-generated diff files (which we have to process) that we wouldn't ever have expected to even consider thinking about before. This all comes from the pain points and lessons learned in that work, and has been a huge help in solving these for us.
These aren't problems end users should hopefully ever need to worry about, but they're problems that tools need to worry about and work around. Especially for SCMs that don't have a diff format of their own, have one that is missing data (in some, not all changes can be represented, e.g. deleted files), or don't include enough information for another tool to identify the file in a repository.
Better file formats cannot, by themselves, improve an inferior SCM tool that, for instance, processes files with the wrong text encoding or forgets deleted and renamed files: they would only have helped you for the purpose of developing your code review tool.
Standards are meant for interchange, like (as mentioned in other comments) producing a patch file by any means and having someone else apply it regardless of what they use for version control.