You make a good argument. I believe the Yankee rule (which may not even be in effect anymore, but I think it is) is because the ownership felt the players should look professional and somewhat clean cut. I agree that being able to express who you are can help motivate you to play better, but I also think that too much 'on-court' personal expression can be a distraction and may subtract from the whole 'team-unity' thing. Something to be said for a guy who leaves his personality in the locker room and goes on the court as a professional concerned about winning. I dont wear dress clothes to work to hide my personality, I do it because it is a condition of employment, like athletes who must conform to uniform requirements.
Of course, there's a limit for everything. When it begins to affect the player's performance, I believe it's time to intervene. It's very hard to draw a line at the beginning, but I don't believe that prohibiting it completely it is the way to go. It's like banning pets because you don't know how to handle them. In that aspect, it may require a couple of tries before you get to the right policy.
That way, I think that the professional look can be summed up to tidiness. But more important than that is having a professional attitude. Because, it's true, they are on their jobs... but they are also on the entertainment business. So, it's required for them to have a certain showman's spirit. And there are good examples of showmanship with extravagant looks. Birdman, comes to my mind. Playing skills opinions aside, I really think he tries to keep the crowd and the team excited while playing around with one of his favorite themes. I don't see how that subtracts on the "team unity" spirit. Au contrair, to be honest, I think it adds a lot. And I think he's tidy. Shirt tucked in, clean clothes at the beginning and leveled socks. For Jordan, this could be a more intimate motivation but still as important.