I mean, the 2 Cave movement is okay on some maps and the 1 Hills is actually useful, but I'm taking Drake Glider all day over Wolf Rider: -1g cost, even better terrain costs (WR still wins Cave but hey, Saurians are more mobile there than either!), dual range including Marksman on the ranged attack which is handy (even if it is weak) to say nothing of superior damage types (Impact/Fire) that may well make up the difference in raw numbers. In other words, Glider is cheaper yet both more mobile and more competent in combat.
As for the other two mid-range scouts, discounting the level zeroes and the 24g Gryphon of course, I'll gladly pay up 1g for the Elvish Scout as it's also more mobile and certainly more combat-capable than either WR or the Glider. (To be fair to WR, again it has better raw damage numbers offensively since the Scout has its total damage output evenly split between melee and range.) While Cavs are slightly less mobile simply on terrain costs, which I dare say really isn't that significant since scouts are first and foremost village-grabbers and vision-providers so the near-universal 1 Flat terrain cost is what rules the day, nonetheless it doesn't come near closing the gap between their relative combat capabilities. Cavs are legitimate frontline fighters.
Or, long story short, compared to the other mid-range scouts, Wolf Rider is the second least mobile and arguably the worst in combat (not strictly worse but tough to argue overall). And it's not even the cheapest among those options! How the hell does that work? Well, I know how it works, because units aren't balanced on an individual basis but by faction as a whole, still they're pretty bad on their own merits.
On strict usefulness, I'm still thinking Bowman although that HI mention is not a bad choice either. Wolf Riders are obviously necessary to operate as a Northerners player. Bowmen, for all their averageness, are simply inferior to Spearmen in most regards even against Drakes. Spearmen beat Clashers head-to-head for their cost, sometimes even outright depending on traits and ToD, and Spearmen are as much better offensively against Burners as Bowmen are defensively. Maybe they fare better against Saurians, since Spearmen's melee damage is mitigated while the Bowman's ranged focus dissuades Augurs from going too hard in the paint. I'm not sure you ever want more than one or two solely for the sake of variety, something you can use as a blocker against range-heavy comps or for the finishing blow on a melee attacker to mitigate damage (keeping in mind that, if you fail to kill, the Spearman is probably better fending for itself defensively next turn).