I think dialects are tough to predict emergent behavior that will inevitably happen if a language becomes old enough. They just want to happen.
If your language assumes this and supports version-dialect from the start it will be more robust and age gracefully in the long run.
edit: basically it would require too much work or a stroke of pure genius to ensure perfect syntax for a new language in public release 1. Language designers should assume as project age approaches infinity the probability of a syntax revision approaches 1.
edit2: also if the interpreter host could interpret all the older versions up to the latest by default, you could write your code in both 2.7 and 3.1 and use 2.5 libraries with it etc. No need to rewrite every single older library for trivial changes.
If your language assumes this and supports version-dialect from the start it will be more robust and age gracefully in the long run.
edit: basically it would require too much work or a stroke of pure genius to ensure perfect syntax for a new language in public release 1. Language designers should assume as project age approaches infinity the probability of a syntax revision approaches 1.
edit2: also if the interpreter host could interpret all the older versions up to the latest by default, you could write your code in both 2.7 and 3.1 and use 2.5 libraries with it etc. No need to rewrite every single older library for trivial changes.