If Yahoo ends up holding the private keys, then it's completely untrustworthy and useless.
There. Did I cover all the bases? Did I leave anyone's pet fear out?
Sorry, let's add a few more things. Let's say Yahoo's CEO is a Scientologist, all their network admins are required to be either Holocoaust Deniers or Creationists, and every employee is required to have at least 25% of their investments in MPAA companies. The receptionists all have iPhones, the corporate mission is the next president of the USA must have either Clinton or Bush as their last name, and henceforth all their web ads will be for either Amway or Herbalife. All the interns are spies for Google and Microsoft and Chinese industries, except for a few which are spies for Mossad, FSB, or Al-Qaeda. The head janitor is being blackmailed by two unknown parties for his participation in a kiddie porn network, and the top sysadmin hasn't heard about Heartbleed yet, the top programmer (who bears the title "Grand Wizard" on his business card) doesn't believe in comments, their implementation of OpenPGP uses a 1938 Luftwaffe cipher as its entropy source for generating session keys, and the company weather station's thermometer was installed on a south-facing patio that gets direct sun all day long.
You may possibly harbor doubts about trusting this company. Yet in that situation, switching to Yahoo email would be more secure than what most people have right now, with plaintext email. So how's that "useless?"