[Exim] conditionally rejecting <> sender

I saw this in a rcpt acl posted in another email:

  deny  message   = Only one receipient accepted for NULL sender
        senders   = :
        condition     = ${if >{$rcpt_count}{1} {1}}

Since there are a lot of people on the list with some very strong feelings
about how email should behave, I was wondering what people thought of
this. It would certainly start preventing spam around here, just not sure
if there's a case I'm missing where that's valid...

--John