Reinforcements is a very tricky card, but also a very powerful one if you build a deck that it slots into.
For trickiness, it basically requires a mono- deck. Not explicitly, but the requirement to pay the resources from 3 different heroes means you need 3 heroes even though they need not be innate if you really want to put Reinforcements in a deck which can only play it via A Good Harvest, Song of Kings, Sword that was Broken, Narya, hero Gandalf or most extreme, Sword-thain. That mono- requirement obviously sharply limits the decks which can use this card at all. But, once again, if you can use it it's well worth it.
So, the use of this card. Well obviously it's essentially a double Sneak Attack, and Sneak Attack is itself an amazing card. So at the most basic, even if you treat this as simply another means of Sneak Attacking Core Gandalf that's pretty good. But there's so much more you can do with this. Obviously as with Sneak Attack Gandalf is the best candidate, but any ally with an enters play effect, a leaves play/discard effect, or any ally which can otherwise make a big difference in a single phase is a good candidate. My review of Sneak Attack brings up many suggestions, but since Reinforcements brings in two allies there's less need to save it for the most powerful allies you have available, since a pairing of allies offers more options than a single ally. Often you will bring in one very powerful ally such as Gandalf, ally Elrond or similar, and then the second ally is of less import - not quite an afterthought, but if you're already getting a powerful benefit out of one ally, the second may as well be whatever you have available.
Availability is one of the other respects in which Reinforcements is very interesting, because as you'll note if you read the text carefully, the allies need not come from the hand of the player who plays the card. You can pick from all allies in any player's hand at the time, which offers a tremendous amount of flexibility. If you're looking for something like Gandalf, the chances of having a suitably powerful ally in anyone's hand are much greater than the chances of having such an ally in one specific player's hand - this of course has always been one of the problems with Sneak Attack, the occasions when you draw the event but not the useful ally to go with it. The allies can also enter play under any player's control, allowing the player to share the benefits. A deck which repeatedly Sneak Attacks Gandalf can easily find it has more card draw and/or threat reduction than it needs, so being able to spread it around is handy. Alternatively one of the very useful aspects of Sneak Attack or Reinforcements is the ability to use them in emergencies, and so this flexibility of Reinforcements allows you to respond to an unexpected combat emergency for any player, not just yourself, without requiring you to bring in a Sentinel ally.
It would be remiss of me not to also mention the combination of Reinforcements and A Very Good Tale as shown off by Seastan. By means of this combo you can replace your temporary allies with permanent ones of similar or equal value. Of course Sneak Attack and AVGT was already a decent combo, but Reinforcements allows you to put into play both the allies you need at the same time, and once again the flexibility of Reinforcements allows you to cherry-pick the best allies (which in this instance means the most expensive allies) from the hands of all players round the table.
All in all, Reinforcements is amazing if you have the means to play it and suitable allies to target with it. Building around it by including a bunch of good ally targets is obviously a pretty good idea, but even without doing that you stand pretty good chances of having worthwhile options in a multiplayer game because generally people will always include some good allies and the only case where they won't be available is if they've managed to get them all into play (in which case you probably don't need the boost from Reinforcements) or if they haven't managed to draw them (which is a separate problem you may want to try and solve). Reinforcements is an incredibly powerful and flexible card which can lead to some of the most interesting plays in this game if you just put in the effort to build a deck which can use it.