Of all the rationalizations that Mr. Bennet uses to justify his decision, that Lydia will not be any worse for going and will even learn her own insignificance is the most specious, IMHO. I find it difficult to believe that he really, in his heart of hearts, believes it.

Does that really sound like the Lydia we know? From what we know of Lydia, is it really likely that being put in a situation with no meaningful supervision and all that temptation, she will behave better than she does in her own neighborhood under her parents' noses? That if faced with being ignored (if indeed she would have been -- it seems to me that girls like Lydia don't have too much trouble getting noticed) she would retreat with her tail between her legs rather than redouble her efforts to be noticed?

I remember reading P&P once with my two sisters, neither of whom were familiar with the story, and both of them predicted at this point that no good would come of Lydia's trip.