The strength of this card seems to come from making the most of powerful actions in the combat phase, potentially abusing that the additional stats will last through the defence and attack steps.
If you're using a cheap ally to ready a more powerful character then you're paying for the readying effect and Miruvor is cheaper (free) spirit option and wont cause a character to exhaust. You really need to abuse the combat phase to make this card worth more than Miruvor.
I'll use Arwen Undómiel because I'm writing this in response to @mduckworth83 's review. If you were to use Tale of Tinúviel with these characters, no other readying effects, you gain no additional stats for the round at the cost of a resource. If you use additional readying effects you get the full benefit of the +3 after the initial one without the additional cost of exhausting a character for it, this is when the card gains some usefulness.
If you use Tale of Tinúviel for it's defence you could have played any number of cheap allies to block instead.
For an attack you could simply have not exhausted Arwen Undómiel and have gotten an additional damage from her, you also save 1 resource and have an additional card in your hand. Instead you could ready whoever your third hero is with Miruvor and gain the use of that character, odds are they have 2 attack and combined with Arwen Undómiel you're getting the +3 you would have from Tale of Tinúviel without having to spend a resource.
I believe Tale of Tinúviel should be used exclusively in decks with additional readying effects, as it's a spirit card there is no reason not to play the event on a character with Unexpected Courage attached at the very least. It's possible that the flexibility of the card is worth the additional cost but what you're paying for is the option to use it for defending as the attack bonus is negligible when compared to Miruvor.
Originally I'd written this because I disagreed with @mduckworth83 's assessment of Tale of Tinúviel, specifically, "any deck that has spirit and a combination of Noldor and Dunedain characters should probalby have atleast 1x copies of this card". I think he's correct because I don't see why anyone would exclude Unexpected Courage from a deck already using spirit.
4/5, should be included if you're able to satisfy it's condition and tweaking your deck to give you an excuse to play it is worth considering.