I think that in the majority of cases a dreadnought wouldn\'t make for a very good leader. From my understanding of the fluff, dreads are not really all there mentality. And spend most of the time in a deep sleep. Bjorn is awoken something like every couple hundred years, or if the Chapter really, really needs his firepower/wisdom/leadership, and this made be the reason that he is for the most part lurictive at these times. Plus think of the poor captain that has been training men, learning tactics, who works best at what, etc., for the last hundred years only to be \"upstarted\" by by some walking tomb. May not be the best for moral.
On the other side, a dreadnought is a link to the history of the chapter, a fount of wisdom and advice, a thing/person of the past, something that isn\'t/ shouldn\'t be wasted. Where\'s living captains/leaders are the present and the future, something that is a little more expendable. And they are still building a name, honor, history, for themselves where\'s a dreadnought has all of these things.
\"Wars should be left the young, because wisdom knows better.\"