Why would they take PMs if the only people who can read them are the owners?
I presume the reason they took the PMs is that they know that many people would have information in their PMs that would help the attacker link them to a real world person. For example: people participating in the FS/T forum very likely would have older PMs containing their name and address.
Consider that the majority of people online are very lazy when it comes to their own security - quite a few people - a significant percentage - use the same password across multiple sites. All they got here was user names and passwords. An automated script, and they can check gmail, yahoomail, etc., for the same username and password. That might not amount to much. But, armed with a REAL name, and password, lazy people under those conditions might find that their financial resources are now jeopardized.
I saw earlier that a few other vB forums have been hit; one for 1.4 million (I think that was the figure) accounts. Think about it - if you're doing all this running scripts, let's say that you link 1% of those accounts to a real name (and I think that's on the low side), and 25% of them are security lazy & use the same password on multiple sites (definitely on the low side) - that's a hell of a lot of bank accounts, logins at sites like Amazon, Paypal, Ebay, etc. Heck, just with the username on the forums, that's STILL a lot of logins at other sites for those who are too lazy to use different passwords.
So, let's say someone got my username here (DrPizza), along with a password of "abcdef" (really, someone used that here as a password) - then an automated script can try DrPizza, password "abcdef" at hundreds upon hundreds of sites in a matter of seconds. For a not so insignificant number of people, that's going to result in a scammer/hacker saying "jackpot!" to himself.