Yeah, they have been first changers, but the point is that they became openers as their talent was recognised. In any case, the real point is that Nicholson is a better bowler than Stuart Clark. Stuart Clark is 30, he's no rookie, and it's unlikely that he will move up to the opening position until Nicholson retires.
By the same token, Mick Lewis is the better bowler of he and Clark. The resemblance to McGrath is imaginary, a combination of similar height and pace do not make him able to hit a good length 6 balls an over.