No, your right, the swarm does constantly change and evolve. I guess kerrigan decided that zerglings needed a bit more depth in addition to the typical swarm attack fighter they have always been. IMO, it adds even more strategy to a loved unit on top of the heaps of existing strategies (like one guy said a while back, zergling drops, the classic rushing, ect) Just as long as they are as fast and in a weird way cute like in the original, I have no problem with the changes (granted i will probably never use the banelings, but thats more about prefrence than anything else).
Oh, on a totaly off topic and derailing point- Has anyone noticed that the new infested terrans use theyre guns now ? I could have sworn i saw a screenshot of them. I wonder if they lost theyre explosive ability from the original starcraft to make room for the baneling ?
Just my thoughts.