I agree opty. I prefer the subject line remain in tact to follow a particular thread. Colorist, you are correct also. Posters need to change the subject line when their messages do not pertain to the continuation of the thread or posters simply need to post a 'NEW' message and title when they begin a new subject. (and I'm including myself. I can do better than I do. I get in a hurry and simply forget to change the title whenever my posts are not on topic or the subject.)
If I'm following a certain thread that I'm enjoying, then I'm looking at the title in order to stay with that particular thread, otherwise, if the title changes, then I wouldn't recognize it as a continuation of that particular thread. I would then be forced to look at each and every post to see if there are any more messages pertaining to that particular thread.
Just my opinion.
Regards,
PxP