I'm not so sure there is a simple answer to it. As many have pointed out, having plainclothes FAM's keeps the "air of suspense" and allows the FAM's to be on some flights and not others. On the other side, a FAM in uniform does present a psychological deterrent.

Personally I'm not all that sure a FAM really does the job. In a perfect world you'd get more than one or two on every flight. But that's not going to happen. I think aircraft security (ie: cockpit doors etc...) is the way to go. Arm the pilots. An armed pilot can do a lot better job from up in the cockpit in than a FAM in the middle of a brawl with more people behind him than in front. But that wasn't the question here.

To answer the question I'd say keep them in plainclothes but go one step further and get rid of the appearance protocol. If i'm looking for a FAM i'm not going to pick the guy in casual clothes with longer hair and two days growth. Kind of like when I was in the Marines stationed overseas and they told us not to stand out while we were out in town. Yeah with a haircut like that??