I wouldn't have thought that it could be claimed that the register was being mis-sold as no-one needs to spend any money before knowing what is available, there has been lots of information out there for quite some time.
It is free to search as many times as required, and is then one's choice to unlock a household (or households) with credits if desired.
By searching the addresses option you can find out the names of the people who lived on a street or road. I was curious as to whether an elderly couple I had lived next door to when I was very young had been there as early as 1939. They were, but unfortunately miss-transcribed so I hadn't found them by name. All cost me nothing to find this out.
As for the merchant navy question, I don't pretend to know, but would they not already be registered when joining the service? The 1939 register was to record every civilian in the land.