Two things: I suspect that not every feature of the client has to do with protocol or repo format interoperability. Secondly, things can change in that regard, too. Secondly, formats and protocols can be subject to versioning and obsolescence also. In short, the situation is not different.<p>&gt; <i>Multiple versions of Emacs can always inter-operate as long as they can all read and write text files.</i><p>What? No; Emacs is a language implementation, changes in which can easily break old code. Sure it can <i>read</i> some elisp file you wrote in 1991, but will it work?