They could be trying to be helpful by using it for two-factor authentication or as a secure means of sending potentially sensitive information. The problem can happen when well-intentioned security advocates talk about how to increase security and Yahoo takes their advice at face value, without taking into account the needs of people who don't need or want increased security, but would benefit instead from decreased security.
"Could" but I don't think so. No land lines will work, which other 2FA providers support. Only mobile, which is almost as good as an SSN because of portable mobile numbers. 2FA is a crutch to get my mobile number. That, or the fact that every advertised workaround has been "broken" for several years straight in the Yahoo registration process is explained by Yahoo simply being inept.
