Spears are supoptimal (not useless) in Warband SP, because you don't need them there. MP is a different story. As opposed to bots that don't use cavalry very effectively, humans do. Without a spear or ranged weapon it's really difficult to counter a human player that plays cav competently.
One handed spear with shield does not exist in a vacuum. You use this to assist team mates and to counter cav while under archer/xbow fire. If you're not under fire, you switch to 2H spear (and decent players typically block well enough for the lack of a shield to not be an issue).
People just play naked in Warband for lulz, if there are good archers on enemy team it's clearly suboptimal to risk additional one hit deaths. If they 'destroy everyone' it's because everyone else on the server is bad, in comparison.