Gathering session cookies with Firesheep

Uh, it protects against all forms of passive snooping of your network links. That's a huge increase in practical security. Not only is an active attack frequently harder to achieve, it also risks detection by the victims.

If everyone's "insecure" HTTP sessions were being encrypted that also makes widespread untargeted monitoring by e.g. a spy agency less feasible. You'd have to put your sniffer in the middle of things, and risk detection. (which I'm sure they do sometimes, but it has to be targeted...) Currently, someone could be sniffing the whole internet and nobody would have any way of telling.