Beautiful and heartfelt episode, tying together all the characters we have grown to care about in new and emotionally gripping way.
While yes it may be seen as a bit of a 'side' episode, and it's a fair criticism in some of the letters being a bit out of character, i think it adds a whole new level of depth to both men, Calvin especially. He was a man searching all his life and firm in his views, yet respectful and open to others.
Without Calvin, no other character in this show would be the same. Nor would the story be the same. It is for everyone's benefit, ours included, that we knew him more intimately.