There was an extensive article in HT mag covering what is tested and how it works, as well as the fact that the manufacturer's don't pay THX for the testing. I'm sure there are plenty of receivers out there that would pass the cert, but they choose not to send them through the process. Why? I don't know. It does mean they need to add the post processing and other features required, aside from just the amplification testing, so not having to add that could mean slightly lower cost. For the numbers they manufacture, having it or not having it probably won't affect the cost to produce significantly.
I think it only matters to a small percentage of the public (bragging rights?), considering the general public doesn't really even know what it truly represents. I'd say most know the name, but if you asked them to explain it, they wouldn't be able to.