A fully improved (5/5) Demoralizing Shout will reduce enemy Attack Power by 420 at Rank 7, instead of 300 with no talent. This works out to be over 8.5 DPS reduction of physical attacks on any enemy players affected, compared with an untalented shout. (See Attack Power for details)
This talent is considerably more useful than Booming Voice, in that it makes the shout more powerful, without making it harder to control. It is particularly helpful in group-based melee furballs, and for long-duration boss fights where the more status effects, the better. (Bosses will also have a larger DPS reduction, as NPC AP affects their total DPS more.) It should be noted that only two ranks (2/5) effectively work against bosses, unless the boss is afflicted with Curse of Recklessness, in which case all five ranks (5/5) are effective.
The Fury Tree shortest path advancement alternative is Unbridled Wrath, which is also a reasonably useful talent. In general, Unbridled Wrath tends to be more useful for soloing warriors (especially those aiming for dual-wielding builds), while Improved Demoralizing Shout tends to be better for group work against multiple targets (and/or builds centering around slow 2H weapons, such as Mortal Strike/Flurry).