They hold some minimum value in that common stock holders are entitled to a share of the liquidation value of the company, should it go under. That said, they'

They hold some minimum value in that common stock holders are entitled to a share of the liquidation value of the company, should it go under. That said, they're (usually, at least) in line behind bond holders, and most companies are trading a high multiple of their liquidation value these days, and companies tend to take on a lot of debt right before they go under, so to a reasonable approximation, the amount of that value that a common stock holder can expect to see is 0.