Curious defaults...

Yesterday, I started using MH at home, instead of elm. For this, I
changed my filter file to pipe the messages into rcvstore, instead of just
appending to a file. However, looking at the headers, I noticed that
their were none related to the envelope, like "Return-Path", like there
usually were.

Closer inspection of the manual, of course revealed the answer that these
are specifically turned off for pipes, but turned on for appendfile's.
Does anybody have any idea why this is so? Is it somehow meant to prevent
messages getting sent out with Return-Path headers?

Cheers for any help,

-Dom