No one says a bulk header would be harmful.
actually, in the past it has been harmful.
- some list expanders used it as a loop-prevention flag - assuming
that any message that had precedence: bulk had already been
fowarded to the list.
- one mail gateway used precedence for 'priority' and would bounce
any message that contained a precedence keyword that it did not
understand. IIRC "bulk" was one of those not understood.
the latter mail gateway is probably long gone (and may have been fixed
before it died) but it wouldn't surprise me if some list expanders
still had that behavior. and the IETF list gets forwarded to lost of
that's what happens when people use nonstandard protocol extensions -
without a widely agreed-on definition of what the extension does,
people will use the extension to do whatever they like - whether it
conflicts with existing usage or not.