This card was largely panned by a lot of reviewers when it came out because it is situational, and unfortunately a lot of players fall into trap of being biased towards thinking only "easy" cards that play well in every situation are good. I personally think it comes from a competitive LCG/CCG mentality but I'll leave that alone for now.
Conditions aside, the effect is fantastic. A +3 stat boost for 0 cost is almost always a really good deal, but the beauty of this card is you get it in attack or defense. While attack boosts are a bit more common, defense boosts tend to be a bit harder to come by. +3 is a good relevant number also because it's just enough to take a non-defensively focused hero (and maybe even ally) to a point where he can answer an attack from most boss enemies and likely survive. What is even better is the flexibility though, you get either/or depending on the needs of the situation, and flexibility is often a quality that's hard to judge at face value and not always terribly apparent when looking at a card in a vacuum. The one draw back is that it only lasts for one attack, but since most characters in the game are not going to have a source of action advantage most of the time, that's all it really needs to do in the majority of cases.
The conditions to play this card are definitely limiting, but that just means that barring certain scenarios, it's a card that is designed to be good in a very specific deck... namely the trap deck. It combos great with Damrod and traps like Ithilien Pit and Entangling Nets because these cards effectively turn into 0 cost markers, that make an enemy eligible to card effects like this one on top of their printed card effects of the trap itself. However, the use of this card isn't only confined to trap decks. There are two other situations where it plays well.
More and more scenarios seem to be including treachery (or even enemy!) cards that turn into enemy attachments. Some good examples are the nazgul and morgul attachments in the Black Riders box, and the Warg enemies that become attachments in Race Across Harad for a couple of examples. This card is definitely a great sideboard card for these quests and others like them.
- Many people probably don't realize this but with guarded cards that are guarded by enemies, the guarded card is considered an attachment on the enemy. This makes this card great with the growing "class" of guarded player card attachments like Necklace of Girion, The Arkenstone, and Orcrist, allowing you to play them and then more easily kill (or defend) the enemies guarding them. Valour of the North is only going to increase in value as more of these "guarded" player cards come out, as well as scenarios that make use of the mechanic on objective cards.
All in all, this card is worth a second look as atleast a solid sideboard item, and you might be surprised how great it works out in a deck built with traps or guarded player cards. It can really be a life saver in the right situation. I hope we continue to see more cards like this, situational but very powerful in their niche situations, as they are what makes the game as well as deckbuilding fun and diverse.