The so-called argument from ignorance is incredibly ubiquitous to every irrational belief out there. And I mean, really, really ubiquitous.
How many times have you seen or heard the stock phrase "science doesn't know everything" as if that were some kind of valid justification for believing whatever that person wants to believe? I have seen it over and over. I have had good friends tell me that (in exactly those words.) The argument is completely nonsensical to justify anything. I really can't understand why so many people think this is such a good argument.
If you get into a discussion with a theist, there's approximately a 99% chance that some form of argument from ignorance will pop up sooner or later (probably sooner.) It can take many forms, the one above being just one of them. Other typical examples include things like "where did the Universe come from?" and lots of claims that start with "science can't explain..."
It's also extremely common among many theists and apologists, that if they succeed in making the skeptic say "I don't know" (or something equivalent), they consider it a victory. As if that were some kind of admission that surely then God must exist, doesn't he?
It really comes down to a false dichotomy: Either you can give a natural explanation for something, or you must accept that God exists. There's no third option.
It's quite unfortunate that many skeptics are actually afraid of just outright saying "I don't know." Personally I don't have the slightest problem in saying it. If someone, for example, asks me "where did the Universe come from?" I just say "I don't know." Why should that be a problem?
(I often follow that up by throwing the same question back. If the answer is something along the lines of "I believe God created it" I ask "and how do you know that?" And so on.)
Skeptics shouldn't be afraid of saying "I don't know." In fact, science and progress thrives when we accept our ignorance. It drives curiosity and makes us try try to discover the explanations, the mechanism behind those things. It's because of the brave people who dared to admit their ignorance that we have progressed so much in science and technology.