It's a false argument that because armor causes a linear increase in TTL (Time To Live) that it actually results in a linear increase in survivability. It's true in absolute terms, but that's not the situation you see in real group PvP (or PvE, for that matter) because of one factor... namely healing.
If World of Warcraft had a healing system like Everquests, Armor would be a linear boost; in that game, the primary healing focused around Complete Heals which would zip a tank all the way up from 1% HP to 100% HP, at least in the original incarnation. If 10,000 armor gives the tank 10 seconds to live without a CHeal, and 20,000 gives him 20 seconds to live without a CHeal, this would be the way to look at the impact of armor.
The problem, though, is that TTL isn't the most important factor in WoW, although it is important... but because of the lack of massive CHeal-style regeneration, what really matters is incoming DPS. If you have 10557 armor, you have 50% mitigation... or someone with 2000 raw DPS to you, they will instead deal 1000 raw DPS; in order for you to remain alive, you have to receive 1000 HP worth of Healing per Second (HPS). If you double the armor to 21,114 you get up to 66.6% mitigation, or someone dealing 2000 raw DPS will drop to 666 DPS... which means you'd have to attack them 50% longer to drop them in absolute TTL terms, or give them 33% less healing in PvP terms.
Armor Penetration provides a correspondingly larger increase to DPS at lower ranges of the armor spectrum. While it may add (say) ~2s worth of life if you exclude heals, requiring the target to receive 10% more healing (due to a 10% DPS increase) when compared to a ~5% increase in required healing (due to a 5% DPS increase) is an immense difference when you _do_ take healing into consideration.