Gilderoy Lockhart is entirely incompetent at magic, the only thing he's good at is modifying the memories of more heroic witches and wizards so he can take credit for their deeds.He set up a Demiguise Derby (keep in mind that the Demiguise is an animal whose hair is used to make Invisibility Cloaks) and got away with a ton of gold. Ug the Unreliable, a notorious goblin con-artist, pulled this in his most famous con.Even Snape, who was the previous potions teacher, would agree.
Slughorn does point out it isn't a perfect solution, and knowing how to make antidotes is still a good idea. note One of the annotations does say "Just shove a bezoar down their throat.", and Harry went with it.
Slughorn rewards him 10 points for "sheer cheek." None of his classmates are amused ( though grateful that he later managed to save Ron from poisoning with it). Harry, who has been receiving help from the Prince's annotations, realizes there's no real foolproof cheat around this (as Hermione also gleefully remarks on), so he grabs a bezoar from the cabinet and presents that as his antidote.
In Harry Potter and the Philosopher's Stone, Malfoy tries to tattle on Harry and Hermione for smuggling a dragon through Hogwarts.