The problem is that when users are working from home or travelling and
(say) using facilities in a hotel, they may be in the habit of simply
pointing their SMTP client at the SMTP server of their home or hotel
ISP and sending mail that way. As soon as the organization publishes
MARID records, such mail will start bouncing if the recipient does
MARID checks.
I'm sorry, I thought you we're making a point other than the "roaming user"
issue. An oranisation that wishes to take advantage of MARID should ensure
that their published policy conforms to their actual policy - this is a
truism. It's an obvious consequence that organisations which are unable to
do this won't be able to make such good use of MARID.
In that case, MARID deployment will simply be stalled. Allowing
per-user records allows those departments which are unwilling to adapt
to be exempted from MARID, and to continue to send mail through any
MTA. Those departments that are happy to change their working
practices (or which already work this way) can be included in the
scheme.
Some kinds of extensibility - like SPFs macros - which allow inclusion of
extra data based on local-part, address the issue you raise.