Spears and maybe polearms having a "reach" quality that gives a defense bonus is not unheard, it works.
Should it be a permanent weapon bonus of being a combat modifier?
If its a combat modifier a spear vs spear combat won't receive the bonus, the opponent is far, but your weapon can reach it thus nullifying the bonus.
I know I always give weird ideas, but, how about using something similar to range penalties of melee?
Maybe melee is also divide in close, mid, and far ranges and you get an attack penalty based on the longer weapon used between you and your opponent (this would require that each weapon have a range category, unarmed and daggers would be Close, polearms and spears would be far, the rest is mid, unless you want extra distances)