My own view:
There is always a risk of cross-contamination in any product. It can occur at the final production facility, or at any other step along the way. A dedicated gluten-free facility can receive ingredients from another facility which are already contaminated when they arrive. A worker (or visitor) to the "gluten-free" facility could bring contamination into the facility on their person.
Personally, I don't worry about shared facilities. You must make up your own mind.