I broke both my legs a few years back and I had plates and pins. They also told me that there is no need to remove them unless they cause a problem. I did have a problem with one of them, so I had it removed but the other has been left in for 13 years.
I also carried on doing taekwondo with plates and pins in both legs, so I don't really think you have to worry about re-injury. I think the general opinion seems to be leave them alone if they're not causing a problem. After all, as lizzwizz said, anaesthetic carries a risk and there's also a risk of infection whenever you're operated on.