Dont's forget that shield block value becomes more valuable as your armor, shield block value, avoidance and shield block rating rise. All of those factors increase the percentage of damage that a successfull block prevents. Oh, and once you have shield block up, it doubles, so it's 200 damage per block there.
On the shield enchant: block value only adds overall reduction (and a bit of threat), it does not provide any EHP while Shield Block is not up.
Only while you have Shield Block up can you guarantee that you will take longer to die due to the plating. It also only helps vs physical, avoidable damage. Not to mention that you need to block atleast 3 times in a streak before that plating helps as much as the stamina, because once you get topped off, the extra ~100 damage you absorbed will just mean 100 more overhealing on top of the thousands pouring in already.