I really like the idea presented in the book about moving in and out of balance, like the serpents in the caduceus crossing and separating repeatedly. This is also seen in the balance of night and day on Earth. Only twice per year, on the equinoxes, are night and day in balance. The rest of the time one or the other is dominant. If Temperance is moderation, then it is "Everything in moderation, including moderation." (Oscar Wilde).
Temperance is the blending, the maturity and wisdom one gains from disparate experiences. It is strengthening one's position by welcoming and assimilating differing points of view. I'm not sure I really understood what Blake was referring to with 'fearful symmetry', or even if I really do now, but I think it is the dark side that is necessary to balance the light. It is the loss of innocence (lamb) that comes with facing and overcoming one's fears (tiger).