Since they're tied to people's account details, address and similar, I'd imagine quite aggressive blackmail opportunities going forward if the data gets to the hands of criminals.

Think postal letter named and addressed, giving your email, and the adult (or other embarrassing) sites you were a member of listed on the letter, along with details of a bank account to make immediate payment to...

Also, you may be able to identify people working for certain high profile orgs (defence contractors, etc) and target them further if you can gleam from URLs they have access to internal systems by specific URL.