An HDMI cable would defeat the purpose. It's not just about mirroring what's on your iOS device on to the TV. It's about using the TV to enhance your experience.
Reading text is best done on an iPad. It's perfect for that. Viewing (and sharing) video or photos are a better experience on the big screen.
Once Apple opens Airplay to developers, tv will become your link between your personal iOS device (each person has their own) and the big screen.
As in the scenario in the original post, you'll be able to beam content from an app to your tv to share it with your friends and family rather than everybody huddling around a little iPhone or iPad.
Apple finally figured it out.