As the maintainer of a mailing-list manager, and as a user of mailing lists, I thought I would chip in.
I think there are uses for all three scenarios: (1) reply to sender (only), (2) reply to list (only), (3) reply to all (sender, list and any CCs).
(1) is useful for private replies.
(2) is useful for replies to the list. It can be particularly handy
for continuing a conversation where the OP, who started the thread,
was not the sender of the mail you are replying to, or if you are
going a little off topic.
(3) is useful for keeping non-subscribers in the loop, or for
providing a direct copy to the sender. It is not true that all MLMs,
or even that all decent MLMs, do recipient filtering to avoid
duplicates of this kind. There are a number of reasons for not doing
so. Two were already mentioned: it's not really possible in light of
things such as BCC and multiple addresses reaching the same mailbox.
Another is that sometimes people have direct copies delivered to
their inbox, but copies via the list filtered into a folder. Such
users want both copies.
When a Reply-To header is set to the list address, all three kinds of reply will do the same thing. I generally think this is annoying, but it does have its uses.
Nevertheless, list admins sometimes feel pressured into using this Reply-To technique, even when they don't want to, because of the lack of support for "Reply List" of mail clients.
The trend seems to be to start including a Reply List feature. In Thunderbird, the "Reply All" button turns into a "Reply List" button when a mailing list is detected, but you can still click the edge of button to open a context menu and choose Reply All. Other interfaces, such as Gmail, I believe, have a "Reply to list" link.
IMHO, a really smart client would recognise if a Reply-To header is set to the list address and make Reply List the default, but still allow a Reply (Sender) feature which ignores the Reply-To header. This could save users a bit of time copying and pasting when they want to reply to the sender of a mailing list with Reply-To set.
At the end of the day, every mailing list is different, because every community is different, and different groups will have different uses for these different features. I think it makes sense to offer them all--it is clear enough what they do--and let people simply not use ones they don't need. A number of users, such as me, would regularly use all three.
Best regards,
Ben.
P.S. I would value a constructive discussion of this topic, without name-calling. Assuming, and much less saying, that people who disagree with you are inferior in some way is not going to get anywhere.