This class does appear in some exception messages and a useful toString() would make debugging errors (in this case, an Ajax request exception) much easier.
A component event handler method returned the value org.apache.tapestry5.internal.structure.BlockImpl@160ba2e7. Return type org.apache.tapestry5.internal.structure.BlockImpl can not be handled. Configured return types are java.lang.Class, java.lang.String, java.net.URL, org.apache.tapestry5.Link, org.apache.tapestry5.StreamResponse, org.apache.tapestry5.runtime.Component.
There are a couple of other implementions of Block that should also implement toString() for the same reason.