Sharing an idea doesn't really devalue it, it increases its value. Keeping ideas private seems sensible -- in that you gain individually, but it doesn't make sense globally. The situation can be addressed with the economic concepts of 'gains from trade' and 'absolute advantage'. It is better for countries to trade freely because they all gain from the different strengths of each. In a similar way it is better for ideas to be shared: each idea can very likely be implemented differently or better by other people -- from which everyone gains.
It doesn't mean that ideas are valueless - just that their inherent value depends on lots of factors (such as your ability/likelihood to execute it).
Releasing one of your ideas does devalue it (because there are now more people with the idea in mind who may execute it before you).
Or in other words: the execution builds the value of the idea. Spreading the idea may devalue any ultimate execution.