It seems to me that the use of string theory in calculations of strongly-interacting matter in condensed matter physics is an example of the theory being on the right track. And then there's the application of string theory to black holes. Have these quieted the skeptics? If not, what's the deal? Could a theory be used in making accurate calculations and still be wrong? Aren't the odds of that happening rather small?
I know very little about this field, so please don't take my answer too seriously.
My impression is that, it's mainly string theorists who are excited about this line of research; condensed matter physicists are mostly skeptical (but many are following the field with interest). I guess the main reason is that these holographic calculations have not yet (as far as I know) given anything which is both new and impressive (from condensed matter perspective). But I have to add that, recently I have noticed papers using holographic methods with only conventional (and serious) condensed matter authors (say, this one). This might be a sign of slow acceptance by part of the condensed matter community...?
My impression is also that these holographic calculations are in no way under control (compared to the original Maldacena proposal) and relies on many layers of conjectures. Given a QFT, I don't think anybody knows how to systematically construct the gravitational dual. People just try to construct space times with the correct asymptotic symmetries (AdS, Lifshitz, Schrödinger, ...), try different matter configurations and then assume that the AdS/CFT dictonary is still valid.
They only try to say certain generic things about a big class of QFTs, using holographic methods, rather than calculate precise quantities for a very specific QFT. But these tools have potential to become very useful for non-perturbative physics if they come under control in the future.
But whether string theory can make precise and useful calculations for condensed matter physics, won't say anything about how correct it is as a theory of quantum gravity (or "everything"), as you seem to imply. What application to black holes do you have in mind? Microscopic calculations of black hole entropy? These calculations show that string theory is consistent as a theory of gravity (there are many other impressive calculations of this sort). Only few people would disagree on the impressive consistency of string theory and no alternative theory has been as successful in this regard. But consistency is not enough to declare a theory as correct.
Well, it is not about "silencing skeptics"... first of all skepticism is not something bad. I am skeptic too... we are speaking about science not about belief systems. The ideas behind the holographic principle have been published, the world took notice and can continue to be skeptic... a publication is not a proof. Then, the "positive" results in condensed matter are not that positive after all... they are just particular cases and there are lots of violation of the holographic principle at least for the domains claimed to be valid too by some "believers". As a conclusion, the holographic principle has a rather limited area of applicability but is by far not as fundamental as some might believe...