[alvarosm]

It is often inexperienced people or "business" guys without any technical background who think they have the billion dollar idea when in fact there are a thousand people who had the idea before who couldn't or wouldn't execute.

So, that advice goes for them: forget NDAs and get good feedback. On the other hand, if you really know what you're doing, you'll know if the idea has a lot of intrinsic value and/or potential for being easily stolen (infrequent and often unpredictable in those rare cases), and who you can to talk to about it.

More often than not, "ideas" are pretty simple and valueless and even the best execution can't do much to keep the business afloat. Many successful ideas come from something as simple as merging several different fields (only an expert in both fields is likely to come up with it), or simply from some enabling technologies maturing enough to have everyone and their dog execute the ideas they had but couldn't build successfully before (tablets come to mind, many mobile apps and so on).

Experienced people would know what to do with their ideas. Who to talk to to get advice, what to keep close to their chest or not. Others would benefit greatly from any advice at all they could get and for that they need to run the very small risk of telling their idea to others.