Grilledfish: There
was a demo for Defender's Quest here on GOG here:
http://www.gog.com/news/online_demo_defenders_quest . Other than that, I think you're reliant on older demos or videos.
Anyway, demos are problematic. While they might be great for the customer, they either require support and maintenance by the devs (which would have to be done "for free"), or they become outdated when the main game (or software in genereal) gets updated (which then might not be a fair representation of the game). As well, they would have to be prepared alongside the main game which takes away resources from the devs. Then there's the problem of Bad Demo, Good Game which might dissuade people from buying the game. Or the opposite Good Demo, Bad Game being misleading and making angry customers. Or, hell, Great Demo, Don't Need to Buy the Game (I did this as a kid and couldn't buy games).
P.S.: I find it interesting you use "on accident".
true those are all problems but not having a demo means the customer only have screen shots and videos to judge.
Unless you decide to pirate the game of course but that is illegal and I am sure gog don't want people pirating the game.
Demo's aren't perfect but it is better than just buying blindly.
Plus even if you do decide to go entirely on screen shots and videos the game might not work on your machine, A demo could help you know if the game would even run on your machine.
Also I guess I didn't find it on accident but it was the only thing I could find when I searched for it.