These issues are trivial compared to the bigger issue at play: the right to read and to write without exception.
That is what is at stake here.
Exits sniffing passwords for plain-text forums is a minor concern compared to the bigger issues at play. I'd like to point out that it's trivial to test for this too...(create some plaintext accounts on forums, post there, then run a honeypot server with your own email address and see if you get a login, do this automatically for each exit) However the time commitment isn't so trivial: If you wrote such a program it would be appreciated, please yell loudly about it on the tor-talk list if you do! Tor works by and large by people following some basic principles, like "if you think it should be done, do it".
You could also see the issues with plaintext passwords on HTTP sites and password reuse as a problem of user education. If you're not up for writing software, why not open a bug and suggest some wireframed ideas for how a UI could look to prevent password reuse? There is a whole bunch of ways to contribute.
There are literally 100's of problems like this Mozilla or Tor could get help with... get involved instead of ahead of time saying "This can almost guarantee that this will do waaaay more harm than good." I hope to see your responsible posting of these concerns to a bug tracker. A lot of what gets done in FOSS projects like Tor really is just a bunch of people who've decided "hey, if I don't do this, who will?".
As for doing more harm than good: I can't possibly see how this does more harm than NSA domestic surveillance.