Yeah, I have had my Andersen / Mor-ryde combo for a year now and have towed almost 20,000 miles with no issues and I'm quite happy with it. When I bought the Andersen I called because I was also confused. In their video demo of the hitch the trailer HAS a Mor-ryde yet the internet (and maybe even somewhere on their website although I can't remember now) was even then a buz with the information that you should use nothing but an OEM pin box. They told me I certainly could continue to use my Mor-ryde but that it wasn't as necessary as with a traditional hitch setup.
They are extremely helpful so I would just contact them direct and ask the question rather than rely on what you read here. If you do, come back and let us know