1) Ok, Google/FB common shares were worth something. Those are extreme outliers in exits, and had ethical founders. But founders have another option if they drive the common share value to nothing - retention bonuses. They can say, ok we will make all the common shares worthless, but you can just give me a huge package as part of the aquisition. So employees can't rely on founders looking after common shares out of self-interest. If you read the article, it looks like that's extremely similar to what happened in this case.
2) All these financial experts can figure out a way to protect their necks without leaving the employees necks under the axe.
3) Real protection would also be voting rights, which generally preferred shares get a lot more of, and then less tangible things like invitation to board meetings, something mere employees accept is absurd to expect. As long as employees agree to sacrifice compensation while being told they're not investors and don't deserve to be treated like one, they're getting hoodwinked.