As usual, I'll take the systematic approach. First, there is the UK Chairmanship's draft declaration:
19. The Conference therefore:The Court's reflection paper on this issue was characteristically diplomatic. However, on the whole it did not appear to have any major objections, at least none based on principled concerns. (They did quite reasonably wonder, though, what this would do to their workload.) Disappointingly, the paper does not advocate that the advisory opinions should be binding. However, it does say something noteworthy about the wider effect of such an opinion:
d) Concludes that, to clarify the respective roles of the Court and national judicial systems, the Court should be further enabled to deliver advisory opinions as follows:
i) For those States Parties that opt in, the highest national courts should have discretion to refer a point of interpretation of the Convention to the Court for an advisory opinion in the context of a specific case;
ii) The national court should set out the facts of the case as well as the question of interpretation of the Convention that the case raises and should give its own view on the question referred;
iii) The Court should give its opinion on the point of interpretation raised; leaving it to the national court to apply this to the facts of the case;
iv) Advisory opinions should not be binding; but when applied by the national court the individual in whose case the opinion was sought should ordinarily have no further right to make an application to the Court in the same matter;
23. The Conference therefore:
c) Concludes that Article 35 of the Convention should further be amended to make clear that an application is inadmissible if it is the same in substance as a matter that has been examined by a national court taking into account the rights guaranteed by the Convention, unless the Court considers that:
i) The national court clearly erred in its interpretation of the Convention rights; or
ii) The application raises a serious question affecting the interpretation or application of the Convention;
44. Despite the fact that its advisory opinions would not be formally binding on the domestic courts, the Court itself should consider them as valid case-law which it would follow when ruling on potential subsequent individual applications. Despite the fact that advisory opinions would not have the binding character of a judgment in a contentious case, they would thus have “undeniable legal effects”. Even though advisory opinions are also not binding under the procedures before the ICJ and the IACHR, these courts nevertheless draw in practice upon their reasoning in advisory opinions in the same way as upon its caselaw developed in contentious cases. (Footnotes omitted)On the whole, the reflection paper resists the analogy with the CJEU's prejudicial question procedure.
Now the final outcome of the Brighton Conference:
12. The Conference therefore:...and that is it.
d) Notes that the interaction between the Court and national authorities could be strengthened by the introduction into the Convention of a further power of the Court, which States Parties could optionally accept, to deliver advisory opinions upon request on the interpretation of the Convention in the context of a specific case at domestic level, without prejudice to the non-binding character of the opinions for the other States Parties;
Now you can see what the States Parties were concerned about. They were worried that they would end up convicted in Strasbourg in cases where they did not appear as a party, but at best as an intervenor. (Cf. par. 40 of the Court's reflection paper.) But surely that shouldn't matter to us, neutral observers?
The Court itself was worried about its workload, but that is a pragmatic and much wider issue that should not affect our analysis of whether advisory opinions would improve the level of human rights protection in Europe.
My friend Max Steinbeis was worried about national judges ignoring these opinions. However, as I pointed out to him, even under the British proposals the advisory opinion procedure would only bar access to the usual individual application procedure to the extent that the national courts follow the Court's opinion. If they don't, then the usual remedy remains open. (Cf. the language from the draft quoted above, par. 19(d)(iv) "when applied by the national court", although the proposed criterion of "clearly erred" is significantly more narrow than that.)
Given the abundance of evidence that the prejudicial question procedure has helped the ECJ promote European Integration, I don't see why the advisory opinion procedure wouldn't do the same for the ECtHR. Ideally, the procedure would be open to all national judges and binding in the case at bar, but I'll take what I can get. That said, I'd be happy to read in the comments why I'm wrong.