The strongest argument is to support a sort of poor man's const-generics. Consider a function like strchr(). This locates a character in a (const) string, and returns a (non-const) pointer to it.
The idea is that you can use this on both const and non-const strings. Call it on a const string, you get back a non-const pointer (which you better treat as const!) But call it on a non-const string, you get back a non-const pointer, with which you can mutate the string. So a single function serves both const and non-const uses.
If casting away const-ness were disallowed, the compiler might conclude that strchr()'s returned value cannot alias the input string, and its optimizations would defeat this design. Anyways that's the original rationale.
It does not strike me as much of a benefit, and certainly not worth the cost of creating confusing semantics (in fact, I would prefer (a true) const to be the default in any language, with something like 'mutable' required to override.)
