The use case is to take data from a surrogate escaped interface and either
filter it out entirely or convert it to a valid Unicode string at the point
of *input*, before letting it make its way into the rest of the
application. For example, this approach permits the Python equivalent of
the Linux console behaviour when it attempts to display the improperly
encoded file name.

This approach is likely to be easier to implement in cross-platform code
than dropping down to the bytes interfaces on POSIX systems and using the
Unicode APIs on Windows.