Here's the context.
We've already discussed the fact that the Deck allows you to be a Mysterious Old Man catalyst.
Fanaelialae says, "It's like saying that gold is abusable because you can hire a mercenary company to dog pile the villain. But that's only possible if the GM allows it. The villain might hire the mercenaries to spy on or even turn against the PCs."
Hemlock says, "I don't think the OP realized that giving players a deck equates to giving them access to functionally-unlimited quantities of gold." In context this refers to "things you can do to dogpile villains."
If you do a random draw of (say) six cards from a Deck of Many Things by a random peasant-in-distress, especially one protected by mid- or high-level adventurers, you'll see a bunch of stuff that the peasant just doesn't care much about (you lose all your possessions and magic items, boo hoo), some game-enders, and some extremely generous gifts of huge rewards. A reward distribution that is roughly "balanced" for wealthy adventurers is skewed steeply toward the positive for penniless paupers and peasants, especially if you have some adventurers around to protect you from the Avatar of Death by simply killing it. (Yes, an adventurer will have to kill two copies of the Avatar of Death because he gets his own copy once he enters the fight, but still, it's pretty wimpy.) I haven't done the math but let's say on average each peasant will get 17,000 gold pieces from a six-card draw. By introducing the Deck into your campaign you are then effectively introducing 17,000 gold per peasant into the campaign, which is plenty of resources to dogpile all kinds of villains. Of course it won't really be just gold--it will be supercharged 9th level peasants with magic items, too. The PCs won't be doing much of the fighting, but then they wouldn't be doing much of the fighting if they hired a mercenary company either. Functionally these two approaches are pretty much equivalent: in one you solve problems by throwing gold at them, in the other you solve it by throwing peasants at them. Since a standard D&D trope involves peasants in distress you are unlikely to be limited by the supply of peasants, hence "functionally-unlimited."
Capiche?