There was a great discussion about this a month or so ago (http://news.ycombinator.com/item?id=2376431). I'd highly encourage you to read the comments that were made.

As Lanzaa pointed out here, encryption without identity validation is worthless. If anyone can MITM your connection without you knowing about it, your encryption isn't doing you any good. That being said, there is certainly value to the "SSH model", where you verify the destination's fingerprint the first time you connect and any time it changes. That at least gives you the opportunity to know if someone is attacking you.