If you are not aware, Yahoo! decided to free up usernames not used over a certain period of time. This means you might be able to get that username (McLovin) instead of "Sorry, this username is taken. Do you want to use McLoving14238576 instead?"
Yes, yes. What a strange move. The first thing it comes to mind is that someone might have used that email address before, so it's "dirty". By dirty I mean it could be subscribed to lots of email lists. Or be the alternative email address accessing some services. For example, think of those services using an email instead of username. People could get an address and go around submitting it to the "Forgot password" forms until hitting one that is worth something - who knows? A NY Times subscription, or access to a porn site.
When asked, Yahoo! commented:
"Our goal with reclaiming inactive Yahoo! IDs is to free-up desirable namespace for our users. We're committed and confident in our ability to do this in a way that's safe, secure and protects our users' data. It's important to note that the vast majority of these inactive Yahoo! IDs don't have a mailbox associated with them. Any personal data and private content associated with these accounts will be deleted and will not be accessible to the new account holder.
“To ensure that these accounts are recycled safely and securely, we're doing several things. We will have a 30-day period between deactivation and before we recycle these IDs for new users. During this time, we'll send bounce back emails alerting senders that the deactivated account no longer exists. We will also unsubscribe these accounts from commercial emails such as newsletters and email alerts, among others. Upon deactivation, we will send notification for these potentially recycled accounts to merchants, e-commerce sites, financial institutions, social networks, email providers and other online properties."