It depends on the use, but I can imagine an organization not wanting a person to be able to access their business e-mail account using simply the username/passw

It depends on the use, but I can imagine an organization not wanting a person to be able to access their business e-mail account using simply the username/password of their personal account. So it will really depend on how 'portable' the linking is: if it's being done by cookies on a particular machine, that's less worrisome than if, once linked, the second account is accessible anywhere the first one is.