For some explaination, let me explain why it depends on your shield block value.
Dodge, parry, and misses are 100% damage evaders, you take absolutely zero damage.
Block is a flat amount of damage removed from the total damage dealt.
Normally, you would think that zero damage far outweighs shaving off a few damage, but ill explain why this is not so.
First and foremost, whenever you block, you CANNOT get a crushing blow on that swing. This makes warriors the easiest class to make uncrushable, as with improved shield block, (and roughly a 22-25% shield block percent) you can just keep using the shield block ability, and be uncrushable for 8 seconds, or two swings, not to mention shaving off your shield block value x 2 in damage, meaning a block value of 200 saves you 400 health every time you use it, not to mention an asston more if one would have crushed you.
The other reason shield block is nice, is because it scales with gear in effectiveness, as you get more shield block value on many pieces of gear. (especially new shields) The 31 prot ability, Shield Slam, is DIRECTLY affected by how much your shield block value modifier is. A warrior with a shield block value of roughly 350-400 can make a shield slam crit for as much as a Mortal Strike Warrior with a blacksmithing epic two hander.
Shield slam is THE highest base threat ability in the game. It serves as a great way to get extra threat quickly, and can also serve as a fantastic way to grind with, in a mixture of +Block Value and crit gear. Ive only gotten up to 218 block value (properly talented, see the second specc in my previous post) and ive crit cloth mobs for 1200 about 20% of the time, in my mixture of block value/crit gear.
Hope this explains a bit.