Even in software that doesn’t thread properly, it’s still faster for me to track down mail with a sensible subject line than mail with nonsensical subjects. I’d far rather see the threading get broken than to have the database proposal still sitting in “Re: apple pie recipe”, to use a real-life recent example.

I guess I’d rate the accuracy of the subject line to be a much higher priority than proper threading, even if broken threading were both more prevalent and not retroactively fixable.

I understand what you’re getting at, however Gmail and Mail.app (and most other email clients) handle threading based on the subject line, rather than the proper way (i.e. using the “In-Reply-To:” header). What I’m talking about is practical for right now. Of course, as with anything, YMMV, but for me being able to track down a recent email related to something I’m currently working on is very important, and the same holds true for most of the people I work with. There’s just not enough short term gain from changing the subject to mitigate the damage done to to a thread for subsequent lookups.

I consider the poor threading in Gmail and Mail.app to be bugs, and ones that will probably get fixed eventually, even if they’re not the highest priority. I’m not going to give up short-term benefits in order to realize a slight long-term gain that only applies against a fixable bug.