What are the compelling reasons to have such a feature?

1. For individuals who prefer to use the keyboard over the mouse (such as
myself):  Because it is more efficient to select a portion of the message to be
copied and pasted via the keyboard than it is to remove my hands from the
keyboard, grab the mouse, and drag it over the text I want to copy. :-)

2.  Individuals who are blind access documents (including email messages they
received) by arrowing through the text.  As they do so, their screen reading
software will speak and/or display in braille each unit of text that receives
focus.  As it stands right now, reading messages with Claws Mail is not
accessible. :-(  

We (Orca screen reader development team) have received several requests to
provide support for Claws Mail as an alternative to Evolution and Thunderbird. 
 And we'd really like to be able to do this, but we need some help from your
team on the accessibility front.

The idea of using the keyboard to mark those portions of a message that should
be quoted in a reply sounds good to me!

It's easy to do this anyway, tab into the message and then use shift -> or <-

It seems I am unable to do this in my installation of 2.10.0 *shrug*

I'm unable to do it as well. (Although I guess the bug kinda gives that away)
:-)

Doesn't seem to work, no, I don't remember we coded that. It's an interesting
feature, although it has to be done in a way that doesn't add to the current
focus difficulties...

I'm sure this used to work for me, but maybe it disappeared and I hadn't
noticed. I frequently use the mouse these days but I'm sure that keyboard
selection worked at some time in the past.

Sorry for the incorrect information.

-- Bugzilla database cleanup --
Hi, 
It looks like nobody has been interested in implementing this feature since the
end of 2007; in order to clean up the bugzilla, I'm marking this WONTFIX.

Features in Claws Mail get implemented on a developer-interest basis: if one of
the core developers codes a feature, or if an external contributor provides a
good patch, the feature gets added. If the feature interests nobody with coding
abilities, although it seems nicer to leave old requests lingering in Bugzilla
and let the submitter hope the feature will be added someday, it's more honest
(and cleaner) to close them as WONTFIX.