[Exim] newlines in $message_body

I'm sure I'm missing something obvious, but is there a good reason why
newlines get mapped to ' ' within $message_body ? I can see why the
remapping of nulls happens, but ...

As this behaviour makes certain filtering operations impossible, I was
wondering if the (comparatively easy to implement) addition of another
variable available to filters called $message_body_with_newlines would
be a plausible thing to do?

-patrick.