I've seen many signs of emotion in JB's flavor of Fae as well as in other author's versions as well as historical descriptions of them so I don't agree that they're emotionless. Mab for instance has been angry, disappointed, satisfied, bored, intrigued, curious, etc, etc... and so has Leah not to mention other lesser fae so I don't agree at all that they don't feel emotion. It may be a sort of detached version of the emotion as compared to human kinds' emotions, but they all seem to be represented to some extent at least.
I agree that they are a sort of yin/yang of contrasts to each other, and must struggle against each other, at least enough to keep a steady pressure against each other to some extent to keep the yin(seelie)/yang(unseelie) shape within the encompassing circle of our perception intact. I don't see it all so much as evil vs good as much as a light vs dark thing. We humans mostly prefer the light, and so we see the seelie as 'good', but without the dark, (which many creatures prefer and indeed NEED) how would we perceive the light?
Oh, and as far as their "boredom" goes, I think it goes beyond simple boredom. The long lives of both branches of the fae has resulted in something more pervasive. I think the word "ennui" expresses it much better. I found a rather good explanation of the word here:
http://www.alphadictionary.com/goodword/word/ennui and here are some snipped out descriptions of it from there.
Meaning: A chronic mental listlessness and disengagement caused by boring interactions with the people, places and things around you.
Notes: Ennui is a special kind of boredom that attacks people who know and have experienced everything in life—or think they have.
In Play: Ennui is likely to attack the man or woman who has everything, and has done and seen everything: "Six months into retirement Jack Uzzi found the ennui unbearable and returned to his job as a parking meter attendant." Ennui is often the price of wealth: "Morris Bedda had 6 houses, 24 cars, two planes, a helicopter, a billion dollars, and more ennui than in all of France."I like all that the word expresses in reference to the fae, they don't have to strive for life or a living, and have everything they could possibly want, and they are incredibly restricted by custom too so humans offer them all sorts of variety that they don't have any other opportunity to indulge in.