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I rewrote the relevant section of the module docs (since they were a bit murky in other ways as well).
Since I didn't answer the question earlier, the main reason a bare raise is permitted is because it's designed to be used to a bare except clause (e.g. when rolling back a database transaction as a result of an error). While you could achieve the same thing now with "except BaseException", the requirement for all exceptions to inherit from BaseException is relatively recent - back in the days of string exceptions there was simply no way to catch arbitrary exceptions *and* give them a name.