Message from Elrond is a tricky card. It's not particularly easy to come up with good uses for it, which is one reason why it is in my opinion rather under-rated. The other reason is that it's a purely co-operative card. While this game is co-operative, most people most of the time will not build their decks co-operatively. Rather they will build to make sure their deck can do the things it's supposed to do and hope any gaps can be covered by other decks' capabilities. Message from Elrond thus loses out because it's something which is unlikely to really work unless you build multiple decks in concert specifically to work well together.

So let's take a look at the effect. You can move a card from one player's hand to another's - a point which a lot of people will forget is that you don't have to be passing a card from your hand to someone else, it could be them to you, or them to someone else. This adds some additional flexibility. The problems are twofold however - firstly just because you can move a card from one player to another doesn't mean the receiving player can play that card; and secondly the card will be shuffled back into its owner's deck if it's still in hand or in play at the end of the round. The first isn't complicated to solve - you just need to have multiple decks in the game running at least one of the same spheres. The second is where it gets a bit more fiddly, because it means this isn't a solution for just not being able to afford your big ally or attachment as they wouldn't remain in play. It could work with Gandalf, Saruman, Galadriel or Elrond (or any other temporary allies in future), with Gandalf and Saruman being particularly good candidates since they're Neutral and can thus be played by any deck. Temporary attachments have consistently been cheap thus far so it might seem more of a waste. Mostly though, this card is good for passing events around.

Before I get into that main use I just want to mention a couple of the side-points. The other significant point is of course that it transfers the card, which can have a reason other than letting it be played. If a situation arises where you need to discard one more card to Éowyn to clear a location or quest stage, or someone urgently needs a card to discard to boost with Protector of Lórien, or with Elven Spear but the relevant player doesn't have any cards then you can pass one over. If someone is using Galdor of the Havens and the last card in their hand is something useful enough to not discard but not currently useful (e.g. Power of Orthanc with no harmful Conditions in play), then Message from Elrond could let them pass that card away, emptying their hand so they can draw six with Galdor. And when the card is shuffled back in at the end of the round, well they can hopefully draw it again by the time they actually need it. Or there might be odd cases where someone really wants a card shuffled back into their deck, like Hidden Cache, or an expensive card they want to bring in with Vilya. These aren't really reasons to put Message from Elrond in your deck, but they are odd useful things you can do with it once it's there for other reasons.

But as I said, the main use of Message from Elrond is to pass events around, and there are a few different categories which could come up:

1. Assuming decks share spheres, this could be a case of expedience - who has the resources for this useful event right now? Give it to them then. This would apply for events like Gildor's Counsel or Light the Beacons where it doesn't matter who plays the event so long as someone does, since everyone benefits equally.
2. Passing events which only work on the deck which played them, such as Elrond's Counsel - if a deck really needs to drop its threat but hasn't drawn any of its own threat reduction then this possibility could be a lifesaver.
3. Messing with probabilities for combos. For example, if two separate decks are both running Sneak Attack Gandalf, then the chances of putting together both halves of the combo will be increased if at least one of them is also using Message from Elrond so that if one of them draws one half and one draws the other then they can combine the two across the table. Or another possibility might be if one deck is using A Good Harvest for Steward of Gondor and another is using it for Unexpected Courage (from out of sphere in each case); the first deck only needs one copy of AGH but probably has 3, and the whole table will probably benefit from that Steward being out early, so if the second deck draws it first then passing it over could be very useful.
4. The further extreme of the previous example, a deck could include copies of a significant event which it cannot even play/doesn't even want, purely so they can be passed over to the deck which does want them. I've done this myself with Hands Upon the Bow, but it's easy to think of other possibilities - Feint, obviously, Sneak Attack as mentioned above and Reinforcements would work for a similar reason, Advance Warning for shenanigans, Strength of Arms for big plays, Stand and Fight or Dwarven Tomb for recursion-focused decks, Fortune or Fate for Caldara, A Very Good Tale for ally swarms, etc. The list goes on. This one has some fairly ridiculous potential if you make it work, because not only are you upping your chances of seeing the relevant event sooner, you are also effectively breaking the "limit three copies per deck" rule. How much can your deck do by playing this event three times in a game? How about if you can play it six times? What if you have additional recursion and can do even more? You'll need to have pretty great card draw to fuel this, but the results are, again, ridiculous.

Message from Elrond certainly takes a bit of thought to make it work really well, but the ceiling for what crazy stuff it can do is really high if you're willing to go the effort of constructing two or more decks specifically to work together around sharing events like this. It's not for everyone I suppose, it's really a card primarily for the more tricksy and shenanigansy players so if you prefer to keep things simple it's not unreasonable for you to give this one a miss, but if you are the kind of LotR player who enjoys indulging in shenanigans from time to time you should really give this card a try!