Everyone knows Eleanor's deal, there's no need to rehash it. In higher player counts and against quests with especially brutal treacheries, she's one of the best insurance cards in the game. In lower player counts or against quests with more typical treacheries, her ability is rarely useful enough to justify the heavy hit to your starting board state from bringing her. Since I mostly play true solo with a smattering of two-handed, I can appreciate her power, but I virtually never play her.
Or at least, that was true until Helm of Secrecy. The helm isn't a super popular card, and I get why. Since it's a 1-of, you can't count on seeing it until you're fairly well established-- and even if you could, you won't be able to afford to drop four resources to replace one hero with another (who, thanks to the printed threat restriction, is likely weaker than the first one) until you're in a pretty strong position. But if the first hero was good enough to get you into that position in the first place, why would you want to replace him or her?
But some heroes really front-load the value they provide. Denethor would perhaps be the most famous example-- often I'll run decks where he's the only leadership hero and Steward of Gondor is the only leadership card. Once Steward is down, he passes his one resource per round to the hero in your "main" sphere or occasionally just uses it to pay for neutrals. There are other examples-- Erestor or Galdor of the Havens in combo decks once they've gotten you to all your key pieces, say. Thurindir, Gilraen, or Widfast if you just want the setup effect.
And in decks like that, I love Helm of Secrecy as a toolsy late-game option. Once Denethor is no longer contributing much, why not chuck him and replace him with a new hero? The new hero will need to be cheap, which means you're probably not grabbing them for their stats but for their ability. And "on-demand treachery cancellation" is one of the most impactful abilities in the card pool. (Plus seven printed threat makes it an easy pivot no matter who you're tossing.) Once your board is established, about the only thing you have to fear is a terrible treachery coming along and wrecking it.
There are lots of other quality "toolsy" heroes you can pivot to. Théodred for resource acceleration, Merry or Galadriel for threat reduction, Either version of Beregond for a clutch defender, Balin for shadow cancellation, Dúnhere for picking off shy enemies, Legolas for ranged attack and progress, Frodo Baggins for the ultimate in hero insurance. But the frequency with which I find myself just automatically grabbing Eleanor even in true solo reinforces how much peace of mind her ability really provides.