But when replying I don't want the full history. I end up with email messages that are thousands of lines long, usually with several threads in each, and finding the individual separate thread, spread throughout the entire quote-upon-quote-upon-quote ... is just not feasible. I'm left trawling through the same email many, many times, trying to extract the bits that matter to assemble them into a sensible, consistent, coherent narrative.

At least, that's what happens when I use any of the standard email clients. but I don't. I use a really old, completely non-standard email client so I can, on every reply, extract the text that matters and assemble it into a single conversation, which I then continue.

Yes, I freely admit I'm unusual, but I find it incredibly frustrating that all the systems other people say "Well, it works for me" are, in my context, unusable. And when you say:

    > Along with that, email clients threading messages
    > for you allows for relatively seamless review of
    > history if necessary.

... you are buying into the fallacy that what works for you should work for any right-minded person. Having clients thread the messages actually compounds the problems for me.