My theory is much like Vodkaman's in that the CoG needs to be below the CoB to maximize stability. It doesn't necessarily remove the roll, but it does help. This explains why taller baits tend to be more stable and shorter baits roll more. Much in the same way that flat sided crankbaits tend have a tighter action but rounded ones have a wider wobble. Here is how I view stability in swimbaits (no making fun of my drawing):
With that in mind, here is my theory on rolling. While at rest, all is good, but when it moves, there are several things going on. The distance between the front and back of the bait will grow and shrink as the side to side movement is created. This has a slight effect on the side to side stability as the "footprint" moves back and forth across the center line. Assuming that the ballast is mainly in the middle section, the weight will move away from the center line, which acts as an axel, causing the bait's middle to drop. As it moves back to center, it stabilizes and then the imbalance shifts to the opposing side. Here is another poor drawing of what i am talking about:
Assuming this makes sense, to Vodkaman's point, it is more than just stability as far as the roll is concerned, but stability certainly helps as it takes more force to pull it off balance and makes it quicker to stabilize at the center. If the movement of the weight across the center line causes roll, then to build on Vodkaman's theory, it would stand to reason that the roll might be reduced if the ballast was greater in the front and rear sections with just enough to stabilize the middle to decrease the amount of weight moving off center. That's just a guess though as I'm sure there could be other impacts to the action.
hope this helps!