I have the feeling that Spirit is on the way to become a no brainer-sphere: you'll always want to include it as a minor sphere if not major sphere in your deck. So far the distinction between the spheres seems as follows:
Leadership: Ressource generation, hero manipulation (adding attributes and keywords), ally manipulation
Tactics: Direct damage, combat effects, damage manipulation
Lore: Healing, drawing cards/player deck manipulation, encounter deck manipulation, threat reduction in the staging area
Spirit: Will Power/questing, countering treacheries and shadow effects, discard pile manipulation, threat manipulation, location control
On top of that Spirit has two of the most powerful cards in the game (Unexpected Courage and Northern Tracker), then it can ressurrect Heroes and allies (Hello Lore?!), it recently got the best card drawing card (Ancient Mathom which puts all similar Lore cards to shame), it will receive ressource generation in Khazad-Dum (Zigil Miner). That just leaves direct damage and healing the only things Spirit is not capable... yet (well, Dunhere can already do direct damage on that account)
Not sure why Spirit gets so much love when everyone agrees that Tactics is the most unversatile sphere and nobody uses Leadership creatures except Snowborn Scout and Faramir. But I see an issue in game balance coming towards us...
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