I don't think all metals are created equal. I'd much rather have titanium or steel in my body than mercury. Is mercury actually a metal, anyway?
I think that things such as rods, pins, screws, etc. are more stable than fillings in the mouth, since their surroundings are more constant. Whereas the mouth is exposed to all different kinds of foods, acids, sugars, hot foods, cold foods, etc. with direct contact.
In my totally unprofessional and uneducated opinion, I think the fillings do more harm than the rod in your back and you would still be better off if you got the fillings removed, as that would be reducing your "total load" of exposure.