The idea may not be, but the moment it becomes a demo it can be worth quite a lot. The story, characters and gameplay can be very valuable, it is how the idea is sold to publishers. This is just like scripts in the movie industry. Most publishers have internal studios, that could possibly be sitting idle. While it is not often done, there is the opportunity for a publisher to move on a developer's idea much faster than the developer could.
The video game industry is brutal, especially with people doing copy cat projects. Look at the current farm games on facebook. The first one was FarmTown, made by a small group. Now some of the facebook powerhouses have released the exact same game with nearly identical gameplay and even similar art (FarmVille from Zynga and Country Story from Playfish).
Or just look at the rivalry between Rock Band and Guitar Hero.
The execution is still worth much more, but I think there is still a need to protect the idea.
Also, it's not like FarmTown is getting destroyed in the marketplace after it's been cloned - they're still the third most popular game on Facebook with 18 million monthly active users (behind FarmVille and Mafia Wars).
Sure, they probably were not the first ones to come up with the idea of bringing a social farm game to facebook either. However, it is clear that once Zynga and Playfish found this idea they could rapidly turn it around too.
It is a bit scary as a small developer to know that you need to share your idea with publishers, but to also realize that they can turn around and dump so much money on your idea that they could produce something much much faster that you cant even hope to compete with them.
It is just a different climate than web based startups. Console game especially are pretty much not possible without publishers, which is not something a 3 man web startup ever really has to worry about.
If VCs were keeping tech teams in house that could potentially jump an idea i think the startup community might see the value of ideas a bit differently.
The Rock Band / Guitar Hero situation doesn't fit your description. Harmonix developed Guitar Hero. Red Octane makes the guitar peripheral and owns the rights. Activision bought Red Octane, and along with it, the Guitar Hero brand. MTV Games bought Harmonix, and with it the developers who then went on to make Rock Band.
This is a situation where those responsible were split in two, not one company looking over the shoulder of another.
The video game industry is brutal, especially with people doing copy cat projects. Look at the current farm games on facebook. The first one was FarmTown, made by a small group. Now some of the facebook powerhouses have released the exact same game with nearly identical gameplay and even similar art (FarmVille from Zynga and Country Story from Playfish).
Or just look at the rivalry between Rock Band and Guitar Hero.
The execution is still worth much more, but I think there is still a need to protect the idea.