Yahoo! - "Wish list"
Yahoo! announced that it will open up email accounts that are inactive since over a year for registration to anyone that applies. Yahoo! is explaining this as a service to give everyone the chance to an Yahoo ID of their choice.
As a lot of organisations and in particular web applications use e-mail addresses as part of authentication and identity management there are a lot of things that can expose Yahoo! e-mail users to potential risks should their de-activated e-mail address be claimed by somebody with bad intentions.
One plain obvious scenario to model against is that e-mail addresses that are publicly known (or can be found out individually) are subject to "theft" by being claimed by third parties. These can then proceed to reset the passwords of their choice.
Since their announcement Yahoo! is trying to retrofit some sort of security control into their process by trying to get the biggest players (Facebook) to implement a new e-mail header for password verification. For that reason Yahoo! pushed an IETF Draft called "Require-Recipient-Valid-Since Header Field".... mid July 2013.
It is not a Question of "IF"
This is merely an attempt at reducing the amount possible damages that will arise by the recently announced move of Yahoo!. There are so many reasons that e-mail addresses can be let dormant but remain important to the owner, especially if used to registration purposes.
It is also not a theoretical matter, password reset functionality is known to be a weak link and stealing identities and stealing e-mail address as the first hop is common.
It is not a question of whether this new Yahoo! move will be abused, it will be.