With data reader support, it's impossible to write a program to read an arbitrary stream of Clojure forms. For example, the following code will fail with the current 1.4.0-beta1 tagged literal support:
It might be enough to require that the read side define a reader for point, but what if we do not wish to require that both sides of the conversation know about the #point tag beforehand? Using the identity function as a default allows the reader to handle the literal form as is even when it doesn't recognize a tag (or even care to translate it).
The change from the current behavior is that missing tagged literal parsers are no longer error conditions.