I agree that this year, Roberson would be better suited to play the 4, but IMO, JB is thinking about next year. If things go as many of us expect, Grant and CJ will be gone. That will leave us McCullough & Roberson. In that combination, Roberson will be the 3. So, by giving him experience in that postion, JB is preparing him for what he needs Roberson to play.
Many people have accused JB of only thinking of the immediate needs, and maybe at times in the past, he could be found guilty of this. But, certainly in the past few years, at least, this is just not true. He has proven to be adept at predicting who will leave and when, and at least make an attempt to prepare the incumbent or find a replacement. It is an imperfect science. Some develop faster than expected, some slower.