I know your question is about bats, but here goes.
We just had an engineer look at our balloon-framed 2 story house. We don't have the written report yet, but he did tell us that in most of these old houses the lathe and plaster served a structural purpose to keep the studs from moving. [Our lathe and plaster was long gone when we bought the house] Also, people back then didn't have as much junk as we do now, so often the old framing is inadequate for the loads our modern lifestyles create.
Our house at one time had a fire in the basement that traveled through the wall bay to scorch the skip board on the roof 40 feet above. As long as the bays are accessible, installing fire blocks is a really really good idea. Blocking any available holes will not only keep bats out, but will help with your heating/cooling costs as well. Our problem isn't bats - it's wasps, but the same principles apply. Replacing rotted wood and caulking small openings is tedious, but necessary if you want to live a bat-free life. Also keeping your screens tightly affixed and in good repair in your gable vents is an often overlooked but simple solution.