The other reviews here just assume everyone knows the knife is bad and tried to defend it, so I am here to write the review of why the knife is unpopular.
If you only own the basic set, you may have so few weapons that you feel forced to take the knife just in order to have enough weapons in your deck. Otherwise, though, while the knife is not an unusable card, it is a hard weapon to like.
If you are a combat character, then a knife would be a ridiculously inadequate primary weapon, you need a weapon that does more than one damage and can be used many times. The only purpose of carrying a knife for such a character is to act as an offhand weapon. The knife is adequate for this purpose, you could certainly get away with making a character like this and have fun playing it. But there are reasons why it just isn't that great.
First, it doesn’t particularly solve a problem. A single shot with the thrown knife just isn’t enough extra ammo to really solve an ammo problem, and the knife, while cheap in absolute terms, is actually a rather expensive way to purchase a single extra ammo. The +1 for the held knife is potentially nice against enemies with odd amounts of health, that would be the reason for holding it, but it still isn’t that impactful on the game. It is only relevant if your other hand is holding a gun with ammo (not your enchanted blade or timeworn brand or switchblade), and it is only +1 – you always have the alternative of just punching the foe if you need to save ammo.
Second, if you have decided to dedicate both hands to holding weapons, you would be better off with a two-handed weapon. They do not publish a lot of low-XP two handed weapons so you may not have the option of never putting any one handed weapons into your deck, but the more campaign expansions you buy for the game the more two handed weapons you will have available and the less likely you are to want a knife whose only purpose is to supplement a one handed weapon.
Third, limited deck space. With only the basic set I was often desperate for any card that would act as a weapon so I might put a knife in my deck for that reason, but now that I have lots of expansions there are always more really cool cards to put in the deck then I have space for, leaving little room for mediocre cards like the knife. And in any case, the number of weapons in the deck is not so large that I can guarantee drawing two of them in my starting hand. Since a combat character is practically crippled without a weapon I would really prefer that every weapon in my deck be a weapon I’d be content to see as the only weapon in my starting hand. A knife just is not acceptable as the one weapon you drew. And if I draw two weapons, instead of one of them being a knife, I would be quite happy to get a second real weapon, use the first weapon until I get more money, then play the second weapon if I run out of ammo on the first weapon.
If you are not a combat character, you could cheaply play a knife to kill rats or as a one shot desperation weapon. The problem is that it takes up a precious hand slot for a weapon you're really planning to never use, you're likely better off putting something useful in that hand and either evading, or relying on your friends to protect you, they will likely do a better job running over to fight monsters than you with your one puny knife throw.