In my opinion open carrying as a way of advocating your second amendment rights is the same as carrying around a bullhorn to advocate your first amendment rights. It's mostly obnoxious and does very little real good for the cause.
Open carrying a MP5 clone, a .22 no less, that offers very little in the way of tactical advantage or self defense is obviously just a stunt for attention. That is not a legitimate reason to carry a weapon. Sure, you have the right to, but as in my example, it's obnoxious. When people pull stunts like this, it isn't bringing (positive) attention to the cause, it's bringing attention to themselves. That's the last thing we need.
The Beatles were wrong, happiness is not a warm gun. It's a new gun.