Users don't need to import `Codec` since its companion provides an implicit `Codec` that is used if none is provided (the `fallbackSystemCodec`, see here http://www.scala-lang.org/api/2.10.2/index.html#scala.io.Codec$). An example that this is working right now is [StartHere.scala](https://github.com/fthomas/scalaz-stream/blob/explicit-implicit/src/test/scala/scalaz/stream/examples/StartHere.scala) which does not import `Codec`.

According to http://docs.scala-lang.org/sips/completed/named-and-default-arguments.html a default argument for an implicit parameter is only used if no implicit value can be found. And since there is an implicit `Codec` in the companion, providing a default would be useless.