I assume you have had cervical flexion and extension films to document the cervical instability. If one of the fusions is incomplete, this could explain the instability. Cervical spine stabilazation surgeries are not uncommon, but if you are questioning the need, then a second opinion is reasonable. You should see a neurosurgeon who specializes in spine surgery, preferably at an academic spine center. This will provide you with more information and allow you to make an informed decision. Good luck