The advice might sound sensible in isolation. But immediately after, Polonus hires a spy to report on Laertes conduct in Paris, and literally instructs the spy to slander Laertes in order to trick people into revealing if these slanders are correct. All this from the man who in the previous scene said "This above all: to thine own self be true, And it must follow, as the night the day, Thou canst not then be false to any man."
Shakespeare quotes should not be considered in isolation, they are always part of a larger context. Taking Polonious advice at face value is like the people taking the "greed is good" or "coffee is for losers" speeches at face value.
Because of the length of the play, the scene with the spy is often cut, since it doesn't directly concern Hamlet and doesn't really lead anywhere (we never hear from the spy again). But without this scene, Polonius advice to Laertes stands alone and is taken in earnest. The scene with the spy shows it is all dishonest bullshit which plays into the larger themes of the play, e.g. Polonius later using his daughter as bait and spying on Hamlet.
Shakespeare quotes should not be considered in isolation, they are always part of a larger context. Taking Polonious advice at face value is like the people taking the "greed is good" or "coffee is for losers" speeches at face value.
Because of the length of the play, the scene with the spy is often cut, since it doesn't directly concern Hamlet and doesn't really lead anywhere (we never hear from the spy again). But without this scene, Polonius advice to Laertes stands alone and is taken in earnest. The scene with the spy shows it is all dishonest bullshit which plays into the larger themes of the play, e.g. Polonius later using his daughter as bait and spying on Hamlet.