Aaaaah, forward compatibility... Here are two interesting excerpts from the EPUB Publications 3.0 specification.

In section 3.4.14: Authors may include the guide element in the Package Document for EPUB 2 Reading System forwards compatibility purposes

In section 3.4.1: The (version) attribute (of the package element) must have the value 3.0 to indicate compliance with this version of the specification.

Let's look now at two bits coming from the Open Packaging Format (OPF) 2.0.1 specification:

In section 1.4.12: the version attribute of the package element is specified with a value of 2.0

In section 1.3.2: In addition, to be processed as an OPF 2.0 package, a version attribute with a value of 2.0 must be specified on the package element. A package element that omits the version attribute must be processed as an OEBPS 1.2 package

In summary, <package version="3.0"> is absolutely needed for an EPUB to be parsed as EPUB3 and EPUB2 absolutely requires <package version="2.0"> since the absence of the version attribute defaults to OEBPS 1.2!!! Conclusion, "EPUB 2 Reading System forwards compatibility" described in the first quote above is absolutely illusory. Even worse, compatibility can be achieved if and only if EPUB Reading Systems deliberately ignore the version attribute on the package element... Ooops, to say the least.