I just edited the Session guide to use bind-dirs. So took the chance to read it over again. I think the following sentence answers your question above:
As I recall, this was the only way I was able to maintain access the config files, but if you already have an existing appVM for Session (or Signal), then this would not be an issue.
The guide actually sets up a persistent SessionID, so is directly relevant to creating a disposable Signal template. I have also set up disposable templates without a persistent Session ID…