So with update 10, they added in more visible stats for warframes, wepaons, and sentinels. A long standing sentinel mod is Steel Fibre (of the sentinel variant) which grants +10% armour per rank.
In update 10, it made it quite clear that all sentinels have 0 base armour. I have equipped on most of my sentinels +50% armour mods. It has dawned on me that 100% + 50% of 0, is sitll 0. Which begs the question, why do we have Steel Fibre for sentinels if it is completely useless? Even more so, as to why this hasn't been brought up before as Steel Fibre (sentinel) has been around as long as sentinels have.