Spearmen get Hyper Body, which boosts your max HP/MP (Maple Story mesos) and that of your party by 60% when maxed. This skill is particularly useful for helping partied Thieves, Archers, and Mages to survive more hits from enemies and/or PQ bosses. They also get Iron Will which gives +20 wep def and +20 mag def for 300 sec. It is basically a nerfed Bless with 100 seconds more duration but gives no accuracy or avoid ability bonus.
Even with this skill maxed; it is not even close to being in the same league as Power Guard and is why Spearmen/Dark Knights in Maplestory are not considered a soloing class. The other difference is weapon options. Fighters and Pages can both choose between unstable weapons with high max damage, stable weapons with higher min damage, fast weapons, slow weapons, long reach, short reach, and so on. Though lowest minimum damage, and with the longest reach but the slowest attack speed but spearmen get weapons with the highest maximum damage.
All 3 jobs have some sames and differences in their skills in game (maple story gold). Spearmen use hyper body, which can also be dispelled, which causes confusion for thieves and bowmen who end up dying from their low hp. Fighters have rage which is very useful but can also be dispelled. It's a very useful skill as some people may get annoyed if you do not use it like it is a precious item. Threaten has to be recast every several monsters and does not effect bosses. It works like rage since it drops the weapon defense of the monsters though it could be useful for mages since they do not have very high weapon defense. Threaten is pretty much useful for training only.