Morn was a fugitive from the law because he broke into a bank and stole money. The fact that it was a liquid that he could store in his second stomach is not relevant. He could have just as easily stored the latinum in a vial in his pocket, or, if he had lived as a human in the 20th century, as a pile of banknotes in his mattress. Whatever the method of concealment, he'd be a wanted criminal.
What Morn's heist and subsequent concealment of the latinum (not the gold-pressed latinum) tells us is that the latinum somehow conveys value. But how? You point out that the paper and ink are not the valuable part of a bill. What they convey is
important though. The serial numbers that certify the bill as legitimate currency are what's important.
So, perhaps the latinum conveys information that certifies its legitimacy as legal currency. Quark pointed out that the little bit of latinum that Morn spit out was worth about 100 bricks of GPL. A brick of GPL is pretty big, the largest denomination introduced, so there must not be much latinum in one brick. Of course, the serial numbers on a bill are only a small part of the ink on it. The rest of the ink is decorative, more or less. Perhaps latinum conveys the "serial numbers" of the gold-pressed latinum currency. Maybe each latinum molecule is uniquely identified somehow, and so only a little bit of it needs to be included in the GPL currency to stamp it as authentic money, legal to exchange for goods and services. Without the latinum, the gold is just gold, and while not completely worthless, is worth less because it's no longer fit to be used as currency. A vandalized $100 bill is not worth $100 anymore, if all the serial numbers on it are missing or illegible.