Similar to Knights of the Swan, this card is straight forward to use - the more of them in play, the better your combined . However, this is a 2 cost card, so the benefit to cost ratios are a little different than Knights of the Swan and Anfalas Herdsman.
Let's look at the multiplicative impact of this card and its passive bonus, passive bonuses being the best types of bonuses in games.
The first Ethir Swordsman you play on the table gives 2 for 2 cost. This is a 1:1 stat to cost ratio, already better than the average 0.57 stat to cost "average ratio" of the 56 ally cards released as of Cycle 9, AP 1 (Wrath and Ruin).
If you can get the third and final Ethir Swordsman on the table, it adds +6 for 2 cost, bringing three Ethir Swordsman to 12 (3 x 4 each). This is a 3:1 stat to cost ratio. One of the best cost investment values for in game.
The downside is Faramir has to exhaust to use his power, which means his 2 will usually not contribute to questing. So 4 cost initially gives you typical +3 (+1 for each hero, if no other allies in play), a better than average 0.75 to cost ratio (especially in Leadership sphere), but not as good as a single Ethir Swordsman in terms of return on investment.
But having a second Ethir Swordsman on the table swings the ratios strongly back in favor of the Outlands swarm. Note however, that it is much more difficult to draw a second Ethir Swordsman in the Outlands use case, vs just drawing "any other ally" in the Faramir use case.