How is it more dangerous to alert people about their pwnage and encourage them to change their passwords? It's easier to attack someone who doesn't know they're being attacked.

I can change my password anyway. How would this website possibly know about all the pwnage going on in the world? It doesn't even give details as to what the databases are, until I provide my info.

For the record, I am not arguing against THIS particular site, which does seem legit based on the other tabs on the site and the kind of things it talks about, but still, these are general things to keep in mind.

Bad guys already have these email addresses. That is the point. And likely the bad guys already know which email addresses you care about. Or it doesn't matter.

Well for one thing you are providing your email address to a website, which might later sell it as a "high quality" i.e. personal email to receive SPAM.

By contrast, when I sign up to a service, I can start out using some throwaway email or a even mailinator.com email. Here, I have to put the ones I care to protect.

This is a bit like those services that say "enter your domain name and we'll check if it is registered" and then front-run you in registering it!