Also I believe you're incorrect about the report. Although it doesn't have legal or jurisprudential value (which isn't necessary for use as evidence), it very much does provide commentary on human rights. Indeed it's very mission "to investigate all violations of international human rights law and international humanitarian law that might have been committed at any time in the context of the military operations that were conducted in Gaza during the period from 27 December 2008 and 18 January 2009, whether before, during or after" would make little sense if it weren't able to define what would constitute a violation of human rights law.
Yeah, I see what you mean, and I'm not really in disagreement or anything. I mean I'm no expert on international law, but I've done a little bit of reading on it, and I have immense respect for practitioners of international law, UN special rapporteurs, etc. You can tell by looking at the treatment of Goldstone (before the recantation), Richard Falk, Ben Emmerson, etc. UN appointees to the UNHRC take a lot of shit from the Israeli and United States military establishments. So, I disapprove of Goldstone's recantation in the WaPo article, but I understand the pressure he, and his family, must have been under.
I only mention that the UNHRC has no legal binding authority, only the Security Council does (and not the General Assembly), because it's actually pretty interesting, and not to mention widely misunderstood (and therefore often the subject of nonsense criticism) how the United Nations works. No organ of the United Nations actually passes 'laws', not even the Security Council. The only thing the Security Council does is pass resolutions consistent with UN charter. Besides heeding the resolutions passed in the General Assembly, the SC is not obliged to take into consideration any other international treaty besides the UN Charter. And the resolutions themselves are basically guidelines issued to states wishing to pool resources to accomplish some shared political objective (sometimes by use of military force).
The Goldstone report, which admittedly I've only read parts of and so you probably are more familiar with it then I am, surely does not hope to "define what would constitute a violation of human rights law." Not in technical sense I mean. As far as I know, IHL as body of international law is defined only in the various international treaties that states have recognized(but not necessarily obeyed) over however many years. Like the Geneva Conventions, or various conventions like the Chemical Weapons Convention. The collection of those treaties is where IHL is defined and the most a UNHRC report can ever do is demonstrate how a particular body of facts concerning the actions or policies of a particular state is inconsistent with IHL.
Anyways, I think that's the gist of it. The Goldstone report is immensely import as a recorded chapter of Palestinian history, and has had a big impact on the public debate, and for that we have the work of the UNHRC to thank.