Nathaniel,
I'd go further and claim that the typical user lacks the
interest/attention to deal effectively with more than 2 reply choices,
which the user will naively think of as "the sender" and "everyone."
Trying to get most users to handle a third choice -- e.g. "reply to
all" vs. "followup" -- is asking at least as much of the user as
expecting them to simply edit out the duplicates by hand.
IMHO, the "typical user" or perhaps the "non-technical business user"
lacks the interest/attention to deal effectively with more than a single
reply choice -- "everyone". I continue to be amazed at the stuff I'm
completely needlessly copied on, and have come to the conclusion that for
these users there is only a single, and usually inappropriate, reply
action.
Nick