If you are a protection warrior casually tanking in low/medium/high-level instances, but not a Main Tank or Assistant Main Tank for your guild in Zul'Gurub and higher instances, you'll want to skip this and invest your talents in something else.
However, if you believe you'll find yourself in situation while you tank/offtank some boss or other tough cratures while the rest of the raid is focusing on killing other creatures, you might want to invest on Shield Specialization. The reason is that Improved Shield Block allows for a Shield Block - Revenge combo, which results in less damage taken for you and efficient aggro holding.
Patch 3.2.0 (2009-08-04): Now provides 5 rage on a block, dodge or parry instead of 2 rage on a block.
Patch 3.0.2 (2008-10-14): Moved to Tier 1, now increases rage by 2 on a successful block.
Patch 1.5.0 (2005-06-07): In addition to increasing % chance to block, it now gives the warrior a 20/40/60/80/100% chance to generate 1 rage on a successful block.
Edit
Patch 3.2.0 (2009-08-04): Now provides 5 rage on a block, dodge or parry instead of 2 rage on a block. 
