In DotA, armor auras do not stack (other auras tend to), I understand primarily due to technical reasons.

Do armor auras in HoN stack? If not, should they?

I lean in the direction that they should. I see it as just a technical limitation, rather than a balancing mechanic, like orbs. Unlike % chance items, where 100+% evasion or 9x insta crits are broken, armor scales smoothly.