In particular, it is possible to use a zero knowledge protocol where a MITM attacker cannot determine if authentication were actually used or not except via changes in user behavior if there are any. For protocols which have a mixture of authed and non-authed usage, this can create a significant risk of discovery for an active attacker.
In any case, authentication is inherently tied up with "identity" which is significant UI and application specific considerations which simply do not exist for ephemeral encryption. TCPcrypt achieves an appropriate layering which separates these concerns while killing content observation by passive attack and increasing the risk of detection otherwise.
Re: stripping, again-- how to handle the absence of encryption is also an application relevant decision. TCPcrypt provides sufficient tools for applications to set appropriate policies.