It's a valid scenario, but I was starting to think quite unlikely. If I get a a new jrandom@yahoo.com address, I would have to know who that account used to belong to and on which online services it might have been used. If I'm in Boston and the old "jrandom" was in Atlanta, I'd have to first figure that out and then figure out what bank he used, and be lucky enough that he had not updated his email there. And websites like banks and other financial services require more than just an email address to get a password reset. You need to answer some "secret questions" etc.
But I grew up before Facebook and other social networking fads. I still don't use those services. So I sometimes forget how easy it is to get a very good life history on someone by just searching their email address, very possibly including the answers to typical "secret questions" like your pet's name, where you went to elementary school, etc. and maybe I can even get some clues about what bank they use.
So it really might not be too far-fetched a concern. Still I think it somewhat unlikely that an email account tied to a lot of social networking activity is itself going to be dormant. But it's possible. Maybe the person has the account forwarded to another address and never logs in directly. Would that count as "dormant" ??
Before issuing an account, Yahoo themselves should be sure it's not forwarded, and search for any associated internet content, especially on social media. If an account has not been used in years, AND internet searches for that account turn up nothing, it might be safe to reissue it.