1) The more sites decide to shut down on the 18th, the easier it becomes for major websites like Google, Twitter, and Facebook to join. It also increases the chances that this is going to be covered by the mainstream media. 2) Perhaps a blackout will give some HNers the final push (and time) to make some phone calls to their representatives. 3) It can serve as a demonstration of what the web will be like if SOPA kick…
One one hand a 24-hour outage would result in loss of a couple of bucks, but on the other, they in fact participated in the IPv6 day, so I'm not sure.