MR initially appears to have updated about the breach using a forum that they had disabled so if you didn't see the thread right away then when you came to the site (forums), they were down. The complaint is fairly justified.
I don't see your chain of reasoning for "fairly justified".
It is not that forum information is breached. It is that passwords and e-mails appear to have been stolen and to the point of another member, if PMs are accessed, then there is personal data in there from former market place sales. So to cover bases, treat this like a personal information breach and nothing less.
The blazingly obvious take-away from this: do not ever send personal data in PMs -- on any BBS. If you need to exchange personal information for for market place sales, then use e-mail to exchange those personal information. If you're concerned about security risks of using the market place here, then use a professionally run marketplace service -- a service that will charge you $$$ for its use.
I'd like to know about PMs as well. Not that I gave out bank details or anything, but my home address has been shared.
Consider a different way to say that: you shared your home address on a PM, and that now might have been broadcast. The obvious solution is to never ever share your personal information on a PM.
[...] then I would have likely started figuring out if or where I've used the same password and began changing it on any site I could still access.
Your tag of "Nehalem" indicates that you have written thousands and thousands of messages here. In the year 2013, I presume that any literate users of computer forums know that every single web login should have a unique password.
Please. If you are so darn worried about the integrity of your PMs, then you should never ever have been using your MR password on any other website. Unique passwords everywhere: there is no substitute -- until SQRL systems get deployed.