i think the first name only is to slow down status match activities. MSC cruise line will give you status based on your hotel status once you supply your hotel credentials with your full name. I discovered that it isn't that easy with Marriott. I had to get a few screen grabs that had my full name and status.
I doubt that Marriott is concerned with MSC or any cruise line, but the more you have to supply to a Marriott hotel competitor might deter the "I taking my business someplace else" spur of the moment crowd