What's much much worse than the spam from the original account holder is the fact that you might receive private personal communications from them. E.g. some long-lost friend could conceivably have you in their address book and choose to get back in touch.

Also, what about other accounts on the web that are linked to the email address? Many web sites allow you to reset your password by proving that you own the email address a user was originally registered with.

This seems like a spectacularly bad idea on Yahoo's part. I can't make any sense of it.