Although I should point out that the main use of the middle node is to prevent linkability. Since you always enter using the same set of three nodes for up to 60 days, if Tor used only two hops an attacker with a high bandwidth exit node that you frequently use could see that that set of entry guards is owned by a single entity, and then link your exit traffic to different websites by your entry guard set fingerprint (which is probably not exactly shared by many people). The middle node takes care of that.