I've been cranking out reasonably full-size games every 8 weeks with a few guys for Blurst.com, so we're in the idea generation phase pretty frequently (Off-Road Velociraptor Safari, Minotaur China Shop, Time Donkey).

As a general statement I think it's fair to say that game ideas are worthless. If you want to get semantic on it, though, I think the problem is that most people think theme/treatment/context ideas are game ideas ("You wake up in an insane asylum with no memories").

If you define a "game idea" as something that clearly states what the player is actually doing, from second to second, then I think game ideas have worth. People who demand NDAs just to hear their brilliance as still full of shit, obviously, but ideas distilled to the brink of execution have merit, and can be bandied about in discussion in a useful way.

Of course, boiling a game idea down to that level of granularity is quite hard, so if you have that done you've done much of the mental clarification already.

At that point you may be talking about a game design, which is more than the idea. Just as there's a long way from the idea of a program to the program itself, there's a long way from a game idea to a game design.