To be blunt, Soldier of Dom Amroth is not good. Statwise, you can get a better ally for the same cost in the form of Veteran Axehand, while textwise Soldier's traits aren't particularly inspiring (Gondor and Warrior are good, but not great) and while his cost reduction does, if you play him in the same phase as another card, make him effectively cost 1, 1 cost for 1 isn't as big of a bargain in where it's the standard versus a sphere like or (before Knight of Belfalas came out, at least) where it's rarer. The main thing that saves him from the binder is quests or decks that incentivize ally swarming, such as the Sailing quests from Dream-chaser. If you take Naralenya as your ship, you can reduce Soldier's cost by 1, effectively making him free if you play another card in the same phase. And, while his stats are still uninspiring (why, why, why, why did they not give him 2 ? Would that really break him? Can anything break Soldier of Dol Amroth?), he's a free body for sailing or exhausting to, say, Taking on Water, something highly appreciated.
Basically, if your deck appreciates additional characters to throw at the encounter deck, you can give Soldier of Dol Amroth a look. Otherwise, leave him in your binder and take a good card instead.