Your trainer is right. Having a protection trained dog is like having a loaded gun, except it can go off without pulling the trigger. It is a huge responsibility, not to mention Liability. Unless you have a lot of experience. What kind of experience do you have?
I know I had one for 10.5 years. It is a tremendous commitment. Some dogs don't have the ability to be dual purpose. You dog has to master advanced obedience before even contemplating bite work..
This is just my $.02.