Divorced from the institution of the Guardianship the World Order of Baha'u'lláh would be mutilated and permanently deprived of that hereditary principle which, as `Abdu'l-Bahá has written, has been invariably upheld by the Law of God.
The World Order of Bahá'u'lláh, p. 148.
Several responses can me made in defense of of the authority of the UHJ:
The Guardian provided the essential role of official interpreter. Who among the Guardianship-without-a-Guardian performs this service? The answer is, of course, that the UHJ does it, and quite often. Is the UHJ authorized, infallible or not, to legitimize itself by coughing up convenient translations of excerpts in the nick of time and saying, "see, this passage is saying this, that, and the other"?
How does the UHJ propose to put forward official translations of scripture without possessing the title of interpreter? Isn't translation all about interpretation?
It's one thing to suggest that the UHJ can perform its function without a Guardian while the Guardian is coming of age (as it would have had to do had it been elected while Shoghi Effendi was a boy). It's another thing entirely to suggest that the UHJ can do its job unerringly without the slightest hope of ever having an unerring interpreter again.
And to those who propose the job of Guardian was completed by Shoghi Effendi: how could the job be done without a Guardian-endorsed translation of the Most Holy Book, especially when such variations in translation as we have mentioned arise?
The reason why the Guardianship is critical to the Bahá'í Covenant is plain as day: the responsibility of authoritative interpretation rests entirely on the shoulders of the Guardian. `Abdu'l-Bahá didn't always spell it out for everyone; Shoghi Effendi should not have had to explain it, but I suppose it's a good thing that he did.
As things stand, there is no Guardian, so there is no final judicial authority in the Bahá'í administration. Nor is there an executive. Nonetheless, nature abhors a vacuum, and power will be awarded to those that fill the void, so we have an infallible UHJ with executive and interpretive authority. Who is there to contest with them? There is no separation of powers, no checks and balances, and no dissent.
If one bases one's response to this question on the writings of the Bahá'í Faith previous to the Will and Testament of 'Abdu'l-Bahá, the answer, I think, would be a qualified yes.
Truth be told, I'm not convinced that Bahá'u'lláh ever had any idea that there would be an institution like the Guardianship. His writings point to the House of Justice (HJ) as a divinely inspired—if not unerring—institution, but he says nothing to even imply the functions of the Guardianship. He doesn't even command his heirs to continue a line of inheritence-based leadership. Rather, he seems to think the HJ would suffice. The credit for conceiving the Guardianship ought to go to his son Abbas Effendi (unless the latter's will and testament was, as some have maintained, a forgery), and Shoghi Effendi ought to get credit for laying out the concept in more detail.
It turned out that the Guardianship was plagued by the same family rivalries that infested the leadership of Bahá'u'lláh and `Abdu'l-Bahá. Perhaps Bahá'u'lláh was trying to move toward a more republican—if not infallible—system.
This was not the only big change that `Abdu'l-Bahá ever made in the name of interpretation (monogamy being an obvious example), if indeed it was his idea.
To summarize, putting aside `Abdu'l-Bahá's Will & Testament and the statements of Shoghi Effendi, one can conclude that the HJ is indeed to be regarded as divinely inspired, and authoritative except in the domain of worship. However, if we admit those two sources, then we must consider the doubts raised by statements expressing the necessity of having a Guardian to act as interpreter and chair of the HJ.