2 for 3 resources is alright, but the returning of 1 resource makes this card similar to Envoy of Pelargir, and so Halfast effectively costs only 2, with a little resource smoothing on top. The extra stats means this is roughly equivalent
Add in a single side quest, and Halfast now looks like the Envoy, but has one extra and hitpoint, and has slightly more flexibility in its resource smoothing. With anymore than one side quest, Halfast is just fantastically cheap. Even mono-Leadership might benefit from his resource smoothing if Kahliel, Hirluin, or any of the heroes that require paying a resource for abilities.
The two downsides are that Halfast is unique, and that it's in Leadership instead of Neutral, as should be expected to limit such a cost-effective ally. Leadership has pretty good side quests to play, though.
If you or your fellowship plays side quests, this card should be showing up in someone's deck, even if just one copy of it for the cheap 2 .