It is not adding "superfluous information" (a term one person used to describe accurate and meaningful subject lines such as the specific example "Payment Sent" in a discussion about a single eBay transaction) to make sure the subject is accurate. It's pretty clear that Gmail has broken the standard of "accurate subject lines" (a real standard described below) that has been around for many years, and makes good sense. It might be a good idea, but it is unfortunate that you have to have inaccurate subject lines in messages in order for Gmail's automatic grouping to work properly.
Meaningful subject lines are much more than a "multitude of people who like to do things their way" (which implies that they ignore standards). The following two links are to email etiquette pages:
Clearly summarize the contents of your message in the subject line. Properly titled messages help people organize and prioritize their e-mail.
"Use a relatively descriptive subject line. I find it really annoying when someone sends me a message with just the word "Hi" in it. That tells me absolutely nothing about the message's content. I'm not suggesting that you sum up your life's story in one sentence, but I do recommend that you use words that will describe the general purpose of your e-mail."
One person complained about "using the Subject header to summarize the entire Body of every new message". That's what you are actually supposed to do!
Another mentioned: "(but both still suffer from the same problem of people changing the Subject for no good reason and using Reply when they should be using New)."
As seen above, in the standard way of doing email, the subject should reflect the content. If you have a single conversation about boats and cars, your subject should be changed to "cars" if the entire body this time is pretty much about cars and the subject line says "boats".
There's also one very good reason to hit "reply" instead of "new": just about every email program (including Gmail) will include the reply-to address in "to" and the text of the previous message (which can be quoted as needed). If you hit "new", you have to rebuild all of this by scratch.
Useful, meaningful subject lines are a great idea overall: they let you look at the subject before deciding to look at further detail in the body. Threading message boards such as Slashdot handle them just fine and don't bust the threads just because participants accurately describe the body with the subject. With the single exception of how it breaks Gmail's sorting, it would be better overall if people paid MORE mind to making the subjects of their emails accurately reflect the body. Maybe Gmail should have found another way to do threads without destroying the usefulness of the subject line. There's also another point to be made that email is a shared experience with other users. If you render the subject line useless in order not to break a sorting algorithm on your own client, this surely does not help outsiders you send the email to who have to put up with subject lines that aren't related to the content anymore.. These outsiders include, of course, other Gmail users. Meaningful subject lines also have incredible value when you print off a single email to take to a meeting, pass around the office, or give to a friend. In summary, using meaningful subject lines is a courtesy that improves the value of information, and recognizes the fact that emails exist outside the context of the fragile subject-line-dependent environment of your own personal client.
Bob: I got that email you forwarded. Thanks!
Tim: No problem!
Bob: One question, though: the subject line had nothing to do with the content. Why's that?
Tim: I can't change the subject line because my email client won't know how to sort emails anymore.