I think what people don't consider enough is that you can choose to discard the cantrip.
That doesn't avoid the problem. Choosing to discard a cantrip means essentially Ghost Shipping the card you would've drawn with it. It's difficult to tell whether you'd rather do that than discard a different card if you don't know what you'd draw. Furthermore, if the card is undesirable, then if you didn't have the cantrip in your deck you would've drawn that card instead and could've discarded it, which is preferable to leaving it on top.
None of this is a reason not to get cards like Grand Market that are actually desirable. It's just a reason why low-power cantrips like Pearl Diver and Pawn, which by default tend to be ineffective but harmless, might actually have a tiny bit of harm beyond the tiny bit of help they give and therefore not be worth buying even with no other options.