Right now, every humanoid skeletons are soldier except that one (because it is from the core set and Warlord was not existing yet).
Also, it is very inconsistent that a skeleton with sword and armor is not a soldier while a stripped skeleton minion is a soldier.
Also, it kinda hinder Warlord that want to play with skeleton soldier by lacking a lvl 2 creature. People might not care much about Warlord, but I like the thematic of a Warlord controlling an undead army. Some kind of Darklord.