Yes, he's in a position of power, and could turn up to his meetings wearing only a cunningly positioned silk scarf, and it would have no effect on the banks' willingness to earn lots of money from overseeing the Facebook IPO.
But when you're in a position of power you also have a responsibility not to be a dick. I realize dress codes etc are a bit different over there in the US, but at work here I normally dress pretty casually.
However, if someone's coming in for interview, I will wear a suit. As I know the candidate's going to turn up in a suit, and it will make them feel uncomfortable if I'm interviewing them in shorts and a t-shirt.
The person in the position of power has a moral obligation not to make everyone else feel uncomfortable or stupid. A great example of this is Queen Victoria drinking from her finger-bowl after one of her guests did - in order to save them from any embarassment or discomfort (http://www.bobssermons.com/sermons/archive/030831.htm).
And yes, bankers have acted arrogantly, and swanned around like they own the place too long. But its hardly a great example to set to just start doing the same yourself as soon as the balance of power tilts.