Improved Shield Bash's usefullness at first glance is to prevent spellcasters from casting for a short while, thus reducing damage or preventing harmful effects to you and your party. The same applies for PvP encounters, for example preventing a mage from Blinking after a Frost Nova. At the same time, it is the best way for a protection warrior to pull a spellcaster back to the main party, since by silencing a spellcaster you force him to switch to melee mode.
Additionally, Improved Shield Bash is one of the most effective threat generators for a protection warrior. The threat caused superceeds that of Sunder Armor, for example, while the rage cost is even lower than Improved Sunder Armor. The only thing that prevents the spamming of Shield Bash on spellcasters and non-spellcasters alike, is its 12 second cooldown.
Shield Bash is a nice combo with Bloodrage, since the 10 rage generated is just enough to make a great opener in threat generation.