What Rannen is saying is that if your morningstar is off balance (i.e. mass unevenly distributed through the striking head, or a slightly crooked haft [insert bent penis quip here]) it ceases to be equally wieldy in all directions. An extreme example would be for the MS to be so off-balance as to have an egg-shaped head. You would be forced to strike with the larger end of the egg to deliver the optimal blow. In effect, it becomes more like an Indian war club:
The same holds true of a more reasonably unbalanced morningstar, if to a lesser extent.
Also, you wouldn't be able to swing it laterally as effectively, without turning it to square up with the opponent. Say your opponent sidesteps your downward swing. You would likely wish to swing at him again, laterally, which can be done with a club-like weapon like a MS by swinging backhanded. This wouldn't be as easy if it were off-balance. In effect, an off-balance morningstar acquires hammer-like attributes, if not to the extent that it would in my example.
I think this is what Rannen means when he says you have to change the grip with a sword, or unbalanced MS - not that you actually shift your palm on the handle, but that you must rotate your wrist to swing on a new vector.
Note: Just to what
extent a slight malalignment would effect your swing I don't know, but I think that, physics-wise, one can safely argue that it
would.