If anything Rook should get more armor. Why the hell does a walking castle get the least armor?
Because he has the largest HP pool. That's what the developers were looking at.
HP / Armor / HP regen / Mana Regen
Weaknesses (bottom 2) are Italicized, Strengths (top 2) are bolded. No formatting is 'normal' (middle)
Level 20 Passive Stats with no items or abilities:
Sedna:
3785 / 798 / 8.3 / 8.5
Beast:
4115 / 1083 / 5.8 / 9.5
Ereb:
4405 / 885 / 7 / 7.8
Oak:
4360 / 875 / 7.6 / 6.4
Rook:
4750 / 658 / 9 / 8.2
The issue, however, is that a difference of one or two regen points is virtually useless: the hp or mana regen by BotF/Narmoths/Banded/Monks or Vlemish far, far outweigh the indivisual demigod's regen which makes "having a high natural regen" rather useless. If you invest with passives then sure, sometimes its okay, but not always. Look at Ereb, he has the second-highest Armor AND HP pool, but lowest in both the regens to "balance". Who is one of the strongest demigods? hmmmmmm. It seems that a demigod's indivisual regen rate matters quite little in determining a demigod's final balance.
Rook does have such a massive HP pool, however, and that is probalby one of the reasons why buffing Armor would have a massive effect. Sedna has a full 1k hp less than Rook at level 20. Boost her Armor by 50 and it won't be as significant as if you boost Rook's Armor by 50.
That said, I do think that Rook needs some help late-level (he's fine early level) and I think that increasing his Armor/level would help with that. Rather than just increasing his base armor.