This has happened before with Admuncher. They both operate along the same lines somehow (local proxy). So I can see how they might butt heads.
Its on you to decide, really. Since you likely have already paid for Admuncher, I can see how that enters the equation. The tech is right to a point....the File shield is still scanning, so its not like you are running around naked. But the WebShield looks for things the File shield does not (i.e., the Web Shield scans all packers by default, File shield does not), so to say you are just as well protected is a leap I would not make.