For threat purposes, block value is debatably the best stat for raiding. The only time block value doesn't really hold it's value for survivability purposes is on slow swinging bosses.
As a raid's dps and threat generation scales upward over time, the stress on the main tank to generate better threat increases as well. The best ways of increasing your threat are through block value, making your specials land more often due to +hit and critting your specials. Tier5 helps you do all of this.