sniffing one node won't help
sniffing a lot of nodes won't help
sniffing ALL of them (or, in practice, at least the great majority) let's you do timing, target, and content analysis: taking the entire firehose of tor exit node packets, inspecting them, profiling them, and drawing the connections
this is obviously not remotely doable for almost all organizations in the world. except an organization with the reach, resources, and abilities of the NSA. would you really be surprised if the NSA was actively seeking and tracking almost all tor exit nodes in the world? i wonder how many tor nodes the NSA just flat out started in the first place on their own initiative to guarantee a sizable chunk of awareness of what is going in and out
such a difficult effort will get you some useful information. not everything, and not all the time. but you *will* find out some interesting things, some of the time
thus, "kinda sorta figure out who you might be"