It seems like including a copy of the specifications would help anyone who discovered it to get up to speed on the language that much more rapidly, assuming they wanted to respond in some fashion. Of course, we're biased toward imagining space aliens are awash is amazing technology, whereas it's equally likely that Voyager might be intercepted many thousands of years from now by some species with very limited spacefaring capability.
Voyager is a tool. Its function should be discernible from its form.