Armor Stat Weight/Itemisation

[swelt]

I'm confused.

http://www.wowhead.com/item=52754 - Shield Enchant, 160 armor
http://www.wowhead.com/item=52768 - Offhand/Shield enchant, 100 Intellect

http://www.wowhead.com/spell=74234 - Cloak Enchant, 250 armor
http://www.wowhead.com/spell=74240 - Cloak Enchant, 50 intellect

http://www.wowhead.com/item=58182 - Tier 11 Valor Reward, ilvl 359 trinket, 1285 armor
http://www.wowhead.com/item=54571 - Halion (T10ish), ilvl 271 trinket, 2282 armor

As it stands, the shield enchant and valor reward trinket both look awful. I realise the proc on the valor trinket is a bit better than the halion one (although not hugely) but that doesn't explain nearly half the armor as a passive.

Two questions:
1) This doesn't add up. There doesn't seem to be a consistent value for itemisation of armor as an enchant. Has anyone seen any indication from Blizzard that it's intended?
2) Has anyone seen any math that indicates point for point how armor items should be weighed against other stats (principally stamina). I used to use a 10:1 rule of thumb (10 armor ~ 1 stam) - not entirely accurate but good enough for at-a-glance.

[Airowird]

As stat value, Armor should be worth 4 Strength etc
As compared value, I'm opting more for a 1:6 rule (1Sta = 6 Armor)