[adrianN]

The exact same dangers exist with normal browsing. Intermediate nodes on your route can do whatever they want unless you use proper encryption. In fact, things like sniffing your traffic are routinely done by $ThreeLetterAgency.

[lorenzhs]

Your argument is a classical example of "correct in theory, wildly misleading in practice". As you can see on https://trac.torproject.org/projects/tor/wiki/doc/badRelays (list isn't updated any more, so the real list is likely much longer) or http://www.cs.kau.se/philwint/spoiled_onions/techreport.pdf, there have been several cases where relays actively interfered with user traffic in a malicious way. Malicious exit nodes are used to MitM connections and sniff sensitive data.

Note that Tor doesn't mitigate the three-letter agency problem, as they can just sniff the exit node's target (I certainly would, there's bound to be lots of interesting traffic there).