I just realized: Bellatrix could literally have saved her master's life.

I'm referring to when she was torturing Hermione; if she had simply used Legilimency on her, she would not only have seen that the sword was fake, but she would also have discovered the trio's entire plot, and she would have alerted Voldemort immediately.

And also before that, when Draco was undecided, if she had used Legilimency on Ron or Hermione, she would have discovered everything immediately as well.

And Dumbledore's entire plan would have been ruined.