It's an Insurance Medical Exam - there is nothing "independent" about it.
Most insurance companies use these exams as an excuse to deny medical treatment or benefits in a workers comp case. And sadly, some of these doctors are insurance company hacks - all they do is examine people hurt at work and send a report to the insurance company (since that is almost all of their business, you can imagine what those reports say - you can go back to heavy lifting, your condition isn't related to your work injury, you don't need the treatment your treating doctor recommended...you get the picture).
Spine surgery (or knee surgery or shoulder surgery) is a serious decision, but don't confuse an IME with a true and legitimate second opinion on whether you need the surgery or if its the best treatment for your condition - it's not.
They can send you to the exam but make sure you provide a copy of the report to your treating physician so he or she can debunk it.