> What benefit is there to hiring someone that could retire pretty
  > much at any minute?

Experience? In addition, some people actually enjoy working, and choose not to retire the instant they can.

  > Or someone who didn't have the foresight to plan for
  > their future in a manner that allowed them to retire at
  > a reasonable age?

This again seems to based on the premise that everyone who can, will of course choose to retire as soon as possible. I don't think that is always the case.