Leadership's inclusion in your deck is entirely dependant on your deck-building philosophy:
Using myself as an example of a deck-building methodology that will likely never take Leadership, I do not build campaign specific or party specific decks. Instead I build investigator specific decks; decks that are a complete package of synergy with themselves and the special ability of the investigator they are paired with, and adhere to a full-combat roll, a full-cluever roll, a full-support, or a half-and-half of the aforementioned rolls.
This philosophy, while good for forming multi-purpose decks, lacks the foresight to include cards like Leadership. On a purely theoretical level, two investigators committing Leadership to each-others test is strictly better than two investigators committing Unexpected Courage to their own test. In that train of thought, you trade away flexibility for max potential performance, probably at too high an exchange rate to be considered a good card.
But the extra icon reveals the main use of this card; as a safety net for your fellow investigators with low . If you know you'll be hanging out with a Finn Edwards, a Silas Marsh, or a similarly stated investigator, Leadership will very likely see play in most scenarios you draw it in. One way to think about it is you save them from having to commit two cards to get the equivalent amount of protection.
This is why, when building a party-specific deck, you should always consider Leadership. It's efficacy increases with the more players in your game, as you'll be in a location with another investigator more frequently. Alongside low individuals, mystics might also appreciate the extra boost, particularly Diana Stanley while she's still stockpiling cards underneath her.
And technically, there is ONE synergistic relationship with another card; Liquid Courage. Liquid Courage is very finicky, as the investigators that probably need the horror healing the most are also the ones that are most likely to fail the test that comes with it. Leadership can thus be used to get one good Liquid Courage test and heal 2 horror off of a 5-6 sanity investigator reliably. Not a bad investment, and right up Carolyn Fern's alley, though Leo Anderson can also take those two cards.