This is just a fantastic card. I wouldn't go so far as to say you should run 3 copies in every deck, but I will say that when I myself am building a deck with , I don't ask myself "Does Ranger Spikes fit in this deck?" I put in 3 copies right off the bat, then remove them later if I find myself short on space and it doesn't fit the idea of the deck. My default assumption is that they're in rather than out.
The reason for this attitude is simple: Ranger Spikes can quite simply negate an enemy. Now there are some situations where it doesn't work out so well, enemies with high archery values that you don't want to leave sitting around or enemies which have forced effects that make them engage you regardless; but in general an enemy which lands on Ranger Spikes will be stuck there as long as you care to leave it there. And, significantly, at -2 . That's the thing that makes this really worth it. If the enemy still contributed its then this card would only be worth for enemies you really wanted to avoid, but at -2 , a lot of enemies may as well cease to exist the moment they hit the Spikes.
According to a search on Hall of Beorn just now, there are 442 different enemies in the game, with obviously varying numbers of them in their respective encounter sets. Now of those 442, apparently 199 have more than 2 . So over half the different types of enemy in the game will contribute 0 on Ranger Spikes and you can freely ignore them for the rest of the game. And of course those enemies probably will be more numerous, so the actual proportion will be more in their favour. If we narrow it down to only non-unique enemies with >2, the number of results drops to 145. But then let's consider further - if a 3 enemy lands on the Spikes, that's still not too bad, it'll just be 1 extra for the rest of the game, that's nothing you can't handle. And the number of enemies with >3 is only 79 total, 47 of which are non-unique. So if we're OK in general with any enemy up to 3 landing on our Ranger Spikes, that's over 80% of the different types of enemy in the game, and the proportion likely goes up if one considers how many copies of each appear in encounter sets. Not to mention some of the bad targets will be unable to have attachments so they'll pass your trap by anyway and leave it still available for a future target. Those are odds I will happily take, and they're vindicated by my actual experiences of playing Ranger Spikes. The number of times I've regretted the Spikes because of which enemy landed on them I could probably still count on my fingers.
There's not a lot more to say. Ranger Spikes isn't really a complicated card that you use in weird combos, it's just simple and good. You throw it in your deck, throw it into the staging area, and most of the time it makes your game easier. I'll take 3, please.