The fact this is "high rated" makes me think this is really good news. I hope you'll share this outside this post or at least "sticky" this post at the top of the thread.
It is really good news. They basically said:
1. We agree with your concerns and we'll remove the passwords while we look for a better solution.
2. Officially, our Windows installers are for Windows users. We won't put up any roadblocks (like passwords), but we also can't officially support non-Windows uses of them. (e.g. No time spent on testing to see whether installer config X or Y gets along better with Wine)
3. No matter how well your tools work, our primary concern with Windows installers is giving Windows users the best possible experience. If we have to choose between keeping non-Windows install scripts working and changing to an installer that makes life easier for Windows users, you'll just have to play catch-up. (i.e. We reserve the right to change things again as needed.)
(To clarify point #3, Gowor said that the whole point of the switch to RAR was to allow GOG install techs to apply small, incremental fixes to the installer while preparing a new release without having to rebuild what could be several DVDs worth of BIN files from scratch. As I understand it, the password was just a poor attempt to get back some of the authenticity verification they lost in the process.)