This seems like a metaphor
Tone is the author's attitude toward the speech. Since we know that Dr. King was the author and presenter of the speech, this was one of inspiration, seriousness, hope, and encouraging. This is not to be confused with mood. Mood is your feelings from the text. So, while the tone may be one of hope, to say it was 'hopeful' would be a mood (if that is indeed what you are feeling when hearing it.)
Hamlet feels that is where a woman has a best chance at being faithful, and where she will cause the least amount of damage. After all, as he tells Ophelia also, "why wouldst thou be a breeder of sinners?"; in a nunnery, she won't have children and bear wicked men-like his uncle-that do awful things.
