I think for OU it should be nonlegendairs because that is what Ubers is for. OU is so people can make a normal team that people most likely would make ingame. It is no fun when people have a team full of legendairs and u have none and they just destroy u with their op power. Please take them out of OU or at least make it more fair.
pplzdude raises valid points, but another thing is that not all legendaries are good. In fact a few of legendaries were even NU in gen 5 and still getting poor usage. I highly doubt this is going to happen.