40 Expertise Skill = 158 Expertise Rating. There is no Expertise Gem, 90% of all Expertise Rating is found on tanking gear (and BT+ Tanking Gear at that) save for a few scattered items, and you'd be using _160 Stat Points_ to lower the impact of _one talent_ of _one class_.
Imagine if Expertise didn't exist, but Warriors could drop 2.4k HP (STA is weighed less heavily) just to move from a 50% weapon chain back to full immunity. Or 320 AP. Or 7.3% Crit. How many do you think would take that, exactly? Because the number damn well isn't very high.