As a blind person, I realize that I represent a small minority
perspective on this topic.  For whatever it's worth, what I'll call
"contextual top posting" works much better for me than traditional
quoting.   With the latter, I have to scroll my speech output through
material that has already been posted before.  I cannot immediately
glance to the new material below, and there is no available mechanism
that allows me to jump between quoted and new material, especially since
quoting conventions vary, even though distinctions between quoted and
new material may be visually apparent.  

Often, there are messages with long quotes at the beginning, nearly the
whole original post.  I don't understand why this is better for sighted
people than reading the original post in its true form by simply
glancing further down the page if a reference to that is needed.  Often,
there are quotes of quotes, and it is confusing who said what, and why
the interaction was necessary to quote.

With topics of interest, I sometimes get so discouraged by the amount of
initial quoting that I give up following the thread.  Sometimes, I press
Control+End (keyboard techniques work better than the mouse) to go
directly to the end of the message in the hope that I can then UpArrow a
few times to get to the beginning of the new material.  Often, there is
a signature of a few lines at the bottom, so this technique ends up not
saving me time.

When following a topic of interest, I sometimes want to participate in
the discussion by posting my comments.  Unfortunately, creating a
properly quoted message, perhaps the combination of current screen
reader and Microsoft Outlook technology, is enough of a chore that it is
usually not worth the effort.  When I've top posted instead as a
practical alternative, criticism usually results from some members of
the list, often in a derisive manner, so I'm reluctant to do that.

The most efficient approach for me, both for reading and writing
messages, might be called contextual top posting.  I understand the
problem of top posting with sentences that do not reference the points
to which one is responding.  That practice does not help me either.  I
generally think it's possible, however, to insert phrases that provide
sufficient context to anyone who has been following the message thread.
For example, one could say "I agree that X is better than Y because of
Z" rather than just "I agree its better because of Z."   Or, one might
begin a sentence with "Regarding the X issue," rather than just assuming
that one knows the point being referenced in the previous message below.

With this approach, I also suggest removing all but the message to which
one is replying with a top post, that is, delete its antecedents if they
happen to be included.  Additionally, if the previous message is long,
e.g., a long code excerpt, then delete unnecessary parts to the
response.  

I hope this perspective is helpful.

Regards,
Jamal