The vast majority of senders are not going to want to configure their
MUAs to know about mailing lists.
Actually, subscribers would _love_ to have an MUA that understands
mailing lists. The current de-facto subscription process---
* send a message to the subscription address, getting all the
request
details into the right format;
* receive a message checking that the request really did come from
someone who can read this mailbox;
* send a response to that message, again in the right format
---becomes much easier for the user with some help from the MUA.
I will admit that if MUAs were to automate all of those details (and
lists were to adopt a uniform interface for these functions) it might
be very popular. Though there has in the past been some reluctance
from lists to do this, out of concern that this would facilitate bots
that would subscribe to lists for various purposes - e.g. to allow them
to spam the list, mine subscriber addresses, etc.
Keith