Roman5: When a publisher/developer doesn't release or doesn't care about a demo to me that basically is saying: "I want you to buy this game just based on this trailer or site/mag preview"
Dark messiah of might and magic had a demo made specifically for the game to highlight the new gameplay mechanics and to showoff the tech - That is the best way to do a demo and leave a good impression
It's not always that easy, especially if the team is small and running in tight shift. All the people needed to create a seperate demo might be needed elsewhere. And game making is more and more project oriented, so as soon as your own slice of the work is done, you move into next, especially with the big companies.
The core problem is in the current business model the companies create games: it leaves very little time to do demos, as all the time is needed to make the actual game.
The problem with Shareware is, that not many games work in the style of Doom or Wolfenstein these days. In those game you still got a very specific amount of levels on a specific episode. If original Doom would be made today, there wouldn't be seperate levels, they'd all be connected.
True, nothing would actually stop the companies make a demo out of it and just close the exits. But the problem might lay in the corporate big wigs, who don't see a profit in demos, they see only the expense.