I believe Spike has an even greater destiny in store. He has assisted all 4 princesses in times of need. He helped Celestial fight off an invasion of Canterlot in comic issue 3, helped Luna solve the mystery of the Phillydelphia Dragontown Arsonist, Helped Cadence save the crystal empire, and is always there for Twilight. He doesn't need an element of harmony, he's better than that.