You say organizing is easy with the internet, but sites like overhear.us only just launched, and we really have no data about how effective they will be. Secret organization is not effective either, because in order to actually negotiate anything serious they would have to come out, at which point they'd be vulnerable.
And my example is about arriving at the best possible economic solution for everyone, not simply assuming that the way company ownership works is perfect and therefore any consequences of that are Good Things. My example is about making sure the workers can excercise their economic power. I am most definitely not saying that a Union should be able to prevent a Board from selling the company, nor even entering into a bad deal. I'm saying that a company should not be allowed to use threats of force to influence the labor market (eg keep it disorganized by blacklisting leaders).
The issue about executives destroying a company is an issue for the board of directors, who fire and hire the executives. If the board agrees to sell their company and the shareholders agree, etc, that is their prerogative as they own the company, not management or the employees. Unions are powerless in this context, which is a good thing.
But I guess the reality is that unions in the US are shrinking fast as they are becoming increasingly irrelevant.