If you add a domestic partner to your policy, you must pay income taxes on the portion of the premium for yourpartner that your employer pays. (By contrast, you aren't taxed on benefits for a legal spouse or children.) So if yourpartner has insurance through his or her company, it's probably best to keep the two separate policies.
But there are other reasons: you could be a beneficiary of a trust or estate or a shareholder or partner in a pass through entity and just now getting your Schedule K-1.